Nah, not wholesale key replacement. That would be a full mechanical re-place (pun!). You can't really replace keys on a vintage horn anyways, only fabricate what you need.
Mechanical rebuild (to me) means refitting the keywork for perfect fit, posts aligned, no looseness anywhere, no binding, square hinge tube ends, no play, etc.
I go into the various procedures involved in painful detail on my website here: http://stohrermusic.com/?page_id=41
This particular procedure and nomenclature only apply perfectly to me, though. Different people call it different things- and do different procedures as well. The only commonality is the problem that is (hopefully) being fixed- imperfect mechanical fit and function.