This is fairly common and I have seen it a lot. There is an important philosophy at McKinsey, which is “Make your own McKinsey”. This directly relates to something like a track change, which is what we're talking about here.
What you need is a good standing in the firm (high performance i.e. score 3 or higher) and support from Partners i.e. someone who will champion your undertaking. The actual process is something you discuss with the PD team but it's nothing too complicated, especially in your example from Implementation Coach to Generalist (especially Generalist in OPS, which used to be my role).
Hope this was helpful. Let me know if there's anything more you'd like to know!