Once the consulting design engineer¹⁾ releases a particular design to manufacturing in the form of the of production files needed to make it, then as far as the design engineer is concerned, that version of the design is finished. This means they are going to be moving on with changes to the design and/or their underlying software tools that were used to produce that design. Barring any specific agreement to the contrary, the production files they made for you so you can build the thing today are no longer their responsibility.

The designer takes responsibility for their design, and the manufacturer owns manufacturing. This means the manufacturer takes responsibility for managing the documents they need for manufacturing.

Don't expect the design engineer to hang onto production files that are no longer relevant from a design perspective once they have conveyed them to you. Don't expect them to be able to regenerate the production files either. Assuming they have held on to the original source files, the software they used may have gone through several updates by the time you make the request, so there's no guarantee regarding how the new files will be regenerated. And since there is no guarantee regarding how the new files will be regenerated, they will need to be verified through another round of testing proto builds, even if nothing actually (or was supposed to have) changed.

Owning manufacturing also means being able to distinguish between a design version's final data and its live data. Final data are things that will never change throughout the life of a given design version, like Gerber files. Live data are things that might change, like build notes, which should see a process of continual refinement, or a BOM, where one generic part may be swapped out for another depending on what's available at the time of manufacturing.

If you think the only thing that needs to change in a design is live data (e.g., the BOM) and it requires a design engineer's input, then you and the design engineer will need to decide whether it makes more sense to have them operate only on the live data or to regenerate everything, then re-test and re-prototype, and re-version the design.

The above can be distilled down to two succinct don'ts:

Don't assume that the consulting design engineer will maintain a history of manufacturing files for you. If maintaining a history of manufacturing files is something you expect your consultant to do, expect to have to pay for the service. Reliable, traceable archiving isn't free, easy, or fun. It's work, and not the sort the design engineer wants to do.

Don't ask the design engineer to just regenerate the production documents they sent you before because you either lost them or don't want to dig them up. Regenerating files that have been previously vetted requires vetting them all over again. In most cases this means a full round of testing and prototype building. A good designer will not release to manufacturing anything that has not been properly vetted. It's their ass on the line if something somehow gets messed up in the regeneration.