Yes. Or at least I think so.

AFAIK, the particular instances of X, Xlib, or glibc for that matter are not touched. Their code is already in memory. Only future instances are affected.

Ordinarily, I think it would actually not be safe. But since X is a network protocol and not a library, and Xlib is actually a client implementation of that network protocol, newly launched X clients (meaing programs that use X) will not use code specific to the new X server. The thing is, Xlib is server-independent, thus generic code, which should work for any X server. So the new Xlib would not make a difference with the old X server.