The request is usually small and well founded. A customer complains about one detail, a component goes end of life, a competitor adds a feature, a regulation moves, or someone finally fixes the thing that has annoyed everybody since the first run. On the drawing it is an hour's work. On a product that is being made, packed, certified, documented, sold and repaired, that hour propagates into six or seven places, and the ones that hurt are never the ones that were discussed.
The written change request
A change discussed in a thread and confirmed with a thumbs-up is how two versions of a product end up on the same pallet. The instrument that prevents it is a single document, short enough that people actually fill it in: what changes, on which drawing and from which version to which, why, who on your side approves it, and from which order you want it to apply. The factory then answers in writing — not with « no problem », which is the answer the question invites, but with the consequences across tooling, component stock, certification, documentation and the date from which they can actually do it. The value of the exchange is almost entirely in that second half, because it is where the real cost of the change becomes visible, and it is visible before you have committed to it.
- Write the request, with the drawing reference, the outgoing and incoming version, and the reason. The reason matters: a factory that understands why often proposes a cheaper way to get the same result.
- Ask for an impact answer that covers each area explicitly — tooling, components on hand and on order, certification, documentation and packaging, and the lead time to the first modified unit. An answer that does not mention an area has not considered it.
- Decide the effective point, and write it as something unambiguous — a purchase order number or a production batch, never a date. Dates slip and then nobody can say which units were supposed to be which.
- Settle what happens to what already exists: the components in the factory's stock, the units already built, the goods in transit and the stock in your own warehouse. Someone pays for each of those, and deciding it now costs a conversation rather than a dispute.
- Issue the new revision of the drawing and the specification, and name that revision on the purchase order. The order is what the factory works from; a revision that exists only in a shared folder has not been ordered.
- Inspect the first modified run against the new revision, not against the old habit, and keep a signed unit as the reference for the version. The first run after a change is the one place where a surprise is still cheap.
Tooling, and the components already bought
- Tooling. Adding material to a moulded part and removing it are not symmetrical operations: taking steel out of a mould is a modification, putting it back is an insert or a new tool. Ask which of the two your change implies before you discuss anything else, because it decides the order of magnitude of the whole conversation.
- Component stock. The factory bought parts against your forecast, and some of them are now obsolete. Who owns them depends on what was agreed, which is usually nothing. Expect to pay for some of it, ask to see the quantity and the purchase date, and consider running them out before the change takes effect.
- Certification. A test report describes the product that was tested. A change to a material, a safety-related component, an enclosure, a power supply or a piece of firmware can mean a retest or a new report, and the technical file behind the marking has to be updated whether or not a retest is needed. Ask the laboratory, not the factory.
- Documentation and printed matter. The manual, the label, the declaration of conformity, the packaging artwork, the product page, the spare parts list and the customs description. Printed items have their own lead time and their own minimum quantity, which frequently makes the paper the critical path rather than the part.
- The installed base. Units already sold still need spare parts, and a change that makes a part non-interchangeable creates a second parts catalogue you will maintain for years. If interchangeability can be preserved for the cost of a small compromise, it is usually worth it.
The certificate and the documents follow the product
The part of a change that is forgotten most reliably is the paper, and it is forgotten because nobody in the discussion owns it. A modification changes the object; the declaration of conformity, the test report, the technical file, the user manual, the label and sometimes the customs classification all describe the object, and they are now describing something that no longer exists. Two of those have consequences at a border or in an inspection rather than in a warehouse, which is why they surface late and expensively. The practical habit is to treat the paper as part of the change from the first request: list the documents that mention the thing you are modifying, give each one an owner and a date, and do not let the first modified units ship ahead of them.
Two versions living side by side
Unless you scrap everything, there will be a period during which both versions exist — in the factory, in transit, in your warehouse and in the field. That period is manageable and it is almost never managed. What makes it work is marking: the version visible on the unit and on the outer carton, so that a warehouse operator can tell them apart without opening anything. What makes it fail is a mixed pallet, because once two versions are in one location nobody can pick reliably and the error reaches a customer. Decide, in writing, which version ships to whom during the transition, whether the old stock is sold through or held for spares, and when you stop accepting the old one at all. The transition also needs an end: two versions kept alive indefinitely because nobody wanted to write off the remaining stock is how a product line acquires a permanent second catalogue.
The revision number, and the change nobody told you about
The only reliable statement of what a product is, is its revision number. Not the photograph, not the sample in the drawer, not what everyone remembers agreeing: the drawing version, named on the purchase order, matched by a signed reference unit and by a line in a short change log that says what each revision changed and from which batch it applies. A product without that has no identity, which becomes obvious the first time someone asks whether a complaint concerns the current version or the previous one. The change log is the cheapest document in the whole file and the one that answers the most questions a year later.
The harder case is the change you did not ask for. A factory modifies something without telling you, and it is usually not deceit: a component went end of life and a technically equivalent one was substituted, a supplier changed, a broken tool was repaired in a way that moved a dimension, a sub-supplier altered a material nobody upstream thought worth mentioning. The person who made the decision found it obvious, and from their position it often was. Three habits catch it. A signed golden sample held by both sides, so that a comparison is possible at all. A first-article check on each new production lot rather than only at the start of the programme, concentrated on the few dimensions and the few components that matter. And a clause in the purchase order that makes any change of material, component, sub-supplier or process subject to your prior written agreement — which will not prevent every substitution, but it converts a discovery into a breach, and that changes who pays for the sorting.
Ask one question before you approve any change: from which purchase order does it apply, and what happens to everything already made, bought or in transit? A modification agreed without that answer is not a modification, it is the beginning of a period during which nobody knows what is in the boxes.