Once you have it you can do whatever you want with it that is legal. The rule of thumb is "original configuration". This is more of an issue with rifles, specifically when they get sporterized the wording says they are no longer C&R, however refinishing is okay. This seems to depend on the seller, however if I was selling a sporterized C&R gun I would lean on the side of caution and sell it as needing an 01 transfer. As for grips I'd imagine you would be fine as long as they are the same or similar configuration.
This is not legal advice, just how I understand it. I think most mods you could make to a pistol would be okay, I can't think of too many C&R pistols people are doing crazy stuff like porting barrels, putting on competition slides, or radically reconfiguring the frames on which could create an issue