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.