Could be an internal change that people on the line would never notice. A stiffer spring perhaps.
Part numbering is a complex issue. GM might probably has an engineering policy in place for identifying part modifications. Not necessarily the part number, but a revision code. [Part numbers usually denote form, fit and function. If one of these changes, so does the part number. If two parts are interchangeable, they often get the same part number but a unique revision code.] Circumventing company policy usually warrants disciplinary action, including termination, at most companies.
The other side of this is checks put in place to maintain configuration control and data integrity. Either more than just these two individuals were involved or the GM processes have some serious flaws. Checking revised documents into any decent document management system would include listing changes, new part numbers, revisions made, etc. If GM doesn't have adequate controls in place and these people are in a position to talk about that, the embarrassment factor might be enough to earn them comfortable retirement for their silence.