Once the consulting design engineer ¹⁾ releases a particular design to manufacturing in the form of a bunch of production files, 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 the underlying software tools that were used to produce that design. The production files that they made for you so you could build the thing today mean nothing to them now.

The manufacturer owns manufacturing.

The designer takes responsibility for their design, and the manufacturer needs to take responsibility for manufacturing. This means the manufacturer needs to take responsibility for managing the documents needed for manufacturing.

Don't expect the design engineer to hang onto files that are no longer relevant from a design perspective (i.e., the production files) once they have conveyed them to you. Don't expect them to be able to regenerate the production files either, because the software they use to do this will 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 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 fixed data and its live data. Fixed 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 will need to decide whether it makes more sense to have them operate only on the live data that you provide them or have them regenerate everything, then re-prototype, and re-version the design. Since reinventing wheels isn't fun, most design engineers will try to make things happen with just a BOM or other live data fix if they can, so don't worry that they will bilk you out of hours and prototyping costs if you ask their advice.

Do not 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 design engineer 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. Especially when, as is so often the case, they are being paid a fraction of what they would be getting if they were a staff engineer someplace.

Under no circumstances should you ask the design engineer to just regenerate what they sent you before because you either lost those documents or you can't be bothered to dig them up. Regenerating files the designer has previously vetted requires vetting them all over again. In most cases that means a full round of prototype building. Any designer worth anything will not release to manufacturing anything that has not been properly vetted. It's their ass on the line if something somehow got messed up in the regeneration.