As with any highly used language you end up running into what I call the COBOL problem. It will work for the vast majority of cases except where there's a system that forces an update and all of a sudden a traffic control system doesn't work or a plane falls out of the sky.

You'd have to have some way of testing all previous code in the compilation (pardon my ignorance if this is somehow obvious) to make sure this macro isn't already used. You also risk forking the language with any kind of breaking changes like this. How difficult it would be to test if a previous code base uses a charbit macro and whether it can be updated to the new compiler sounds non obvious. What libraries would then be considered breaking? Would interacting with other compiled code (possibly stupid question) that used charbit also cause problems? Just off the top of my head.

I agree that it sounds nonintuitive. I'd suggest creating a conversion tool first and demonstrating it was safe to use even in extreme cases and then make the conversion. But that's just my unenlightened opinion.