X11R6 is considered a major update from X11R5, so the shared
libraries in XFree86 3.1 and later are not compatible with XFree86 2.1.1
and older libraries. To continue to run X11R5 applications, you must keep
the old libraries somewhere on your machine. They can be moved from
/usr/X386/lib elsewhere, but /etc/ld.so.conf must be updated.
All X11R5 applications should work with the X11R6 servers without problems.
X11R6.1 is yet another update to X11R6. While the minor number for some libraries has been increased to '1' it is believed to be fully compatible with X11R6 based applications.
X11R6.3 is yet another update to X11R6.1. While the minor number for some libraries has been increased to '3' it is believed to be fully compatible with X11R6 based applications.
Very old binaries (linked to XFree86-1.2, XFree86-1.3 or
XFree86-2.0 libraries) will continue to work, but may need an explicit
symlink from /lib/libX{11,t,aw}.so.3 to
/usr/X386/lib/libX{11,t,aw}.so.3.