When Does a Product Change Require Retesting?
A change needs retesting when it could alter how the product meets a rule, and the answer comes in four levels, not two. Most changes a factory proposes mid-order sit somewhere between a paperwork review and a full return to the lab, and how small the change looks on the quote tells you nothing about which one you face.
| What Changed | Usual Level | What You Collect |
|---|---|---|
| Color or colorant | Lab assessment | Colorant data sheet |
| Coating or paint | Lab assessment | Coating supplier sheet |
| Plastic or resin | Assessment or extra tests | Resin data sheet |
| Battery or power supply | Extra tests, often full | New part specification |
| Wireless part or firmware | Extra tests or full retest | Module or software notes |
| Factory or process | Assessment, sometimes full | Written process description |
| Use or marketing claim | Rule check, then tests | New claim in writing |
Get the third-column paper from the factory, pass it on to the lab, then make the call yourself; the lab advises, you answer for it.
The Four Levels, From Cheapest to Most Expensive
Every change lands on one of four answers: file review, lab assessment, added tests, or a full retest. A file review is a short letter from the lab saying the existing report still covers the change. An assessment means the lab reads what moved and tells you which tests, if any, need repeating.
Added tests repeat only the parts of the original testing the change could affect. A full retest starts over. It is common for changes to power, materials that touch the user, structure, or a radio. A new factory, process, or source for a critical part can also send a product back to the start, and for children’s products a change of manufacturer often does.
A quality control partner can log every proposed change and get it answered before the factory moves.
Choosing a good lab does not move responsibility to the lab, under US or EU rules, and it does not move it to the factory either. The lab cannot tell you whether the saving is worth the risk or whether your customer will wait. Those calls are yours; they are what China import compliance comes down to on every order.
The Two-Cent Substitution
The most expensive change in manufacturing is the one the factory offers as a saving. A resin two cents cheaper per unit, a new coating supplier, a component swapped for a look-alike from another vendor: each arrives with the line “performance is the same,” and each can quietly detach your product from its test report.
A substitute can pass every check the factory runs and still fail the one the lab ran. A production line sees fit, function, and looks; the report covers what is in the material, how it burns, and what it does when something goes wrong. A look-alike part carries none of that evidence.
Same base plastic does not mean same colorant, pigment supplier, or coating recipe. Any of those can bring in a banned chemical the original test never saw, so treat a color change as a material change until the colorant data says otherwise. The six colors you order on day one are a different question, and testing every color and size answers that one.
The factory’s usual defenses do not change the decision. “Same material, different supplier” still needs the new supplier’s data sheet, or the swap waits. “It is an improvement” is still a change; a thicker wall or a brighter LED can shift the tested result as easily as a cheaper part.
So put a price on the two cents. Get the retest cost and the delay, set them beside the saving, and decide: accept the swap, refuse it, or accept it only if the factory pays for the testing. The choice is yours, made before production, not the factory’s, made during it.
Changes You Did Not Ask For
The dangerous change is the one you learn about at inspection, or never. Factories swap components when a vendor runs short, move molding to a sister plant when the schedule is tight, and adjust settings to lift output; none of it feels like a change from where they stand. That drift is what turns an approved sample into a different product on the way from prototype to mass production.
The defense is a clause in the purchase order that any change to material, part, source, process, or software needs your written approval first. The clause names the tested build, which is nothing more than the exact parts, materials, and settings of the sample the lab passed, and attaches that parts list and sample.
That is why a China purchase order is worth writing properly: it settles what “the same product” means, turns a silent substitution into a breach, and decides who pays for the retest.
Then look for it, knowing what looking can and cannot do. An initial production check that compares early units against the approved sample catches visible swaps: a different label, a different connector, a part that does not match the photo.
It will not see a changed resin recipe, a different coating vendor, a firmware build, or an internal component with the same shell and a different maker. Those need other tools: the factory’s change record where it keeps one, a firmware version check, the purchase record for the part, opening up a few units, or, where the report is at risk, a test.
A during production inspection catches visible changes that start later; no inspection makes payment safe on its own.
How to Decide in Three Questions
Run any proposed or discovered change through these before you agree to it:
Does the change touch a part, material, or setting the original test measured?
Could it change how the product could fail, overheat, leak, break, or reach a child?
Does it change what the product claims to do or who it is for?
Three noes point to a file review; one yes means production waits for the lab’s answer.
The Coverage Claim
The claim that will not die is that the report covers the whole series, whatever the factory builds. Check it the same way you would verify a test report: read which models and builds it names. If the changed build is not there, hold the balance payment until it is either named on a report or the lab confirms it does not need one.
FAQ
Q1: Does a change I asked for myself follow the same process?
Yes. A new feature, a stronger hinge, or a different finish requested by your own designer moves the tested build just as a factory swap does. It goes through the same three questions before you sign the revised drawing.
Q2: Does a new packaging supplier mean a retest?
Often a file review, unless the packaging touches the product in a regulated way, such as food contact or a child-resistant closure. Have the factory state what changed and check whether your report or label mentions the packaging.
Q3: Should the retest use hand-built samples or units off the production line?
Either, as long as the sample matches the final build in materials, parts, and process. The rules ask for a sample that represents production, not one from a particular machine, so get the factory to confirm the two match before you book.
Q4: Does the retest have to go to the lab that issued the original report?
No. The original lab has your file, which saves some back-and-forth, but that does not mean a lower quote or that it can run the added tests. Any lab needs the original report, parts list, and approved sample before it can say what to repeat.
Q5: Should I trust the lab when it says partial testing is enough?
Yes, when it comes in writing from the lab that holds your file and names the tests it will skip. Keep that letter with the report; a regulator or platform may still ask for more, and the responsibility stays with you.
Q6: Do I have to update the declaration or certificate after a retest?
Yes, and the lab only reissues the test report. The “this product complies” paper is signed by whoever your market holds responsible: often the importer in the US, usually the manufacturer in the EU, which can be you if it is your brand. Then check that report, declaration, and label all show the same name, model, and version as the goods.
Q7: Can the factory keep shipping the old build while the new one is being assessed?
Yes, if the old build is still the tested one and nothing else on the line has moved. Agree a start date for the new build, keep the two in separate lots with their own carton marks, and hold the new build until its retest is done.
Q8: What if the change happened months ago and I only just found out?
Treat everything built since as unconfirmed. Get the lab’s level for the change now, hold the affected stock in the meantime, and read your contract for who pays, which usually turns on whether the factory told you.
Conclusion
A product change you did not price is a shipment you may not be allowed to sell, and by the time you find out the balance is often already paid. Every step above exists to prevent that: money out, goods in, and no legal way to sell them.
Collecting the change record and the written level can be done from your desk. Confirming that the units being packed are the build those papers describe, and catching a swap nobody mentioned, cannot. We do that part as your quality control partner, so a change reaches you as a decision to make before you pay, not as a finding after the container is gone.
