I believe that the client-side default for this is embedded in libpq.so. Probably the way things are set up, the 9.2 psql uses the 9.6 libpq.so if it's installed. That would tend to happen if the libpq.so is being picked up from /usr/lib(64) rather than a version-specific directory ("ldd" would help you check that).
It's pointing to the newly installed version instead of the version it the 9.2 lib directory. There isn't any such file under /usr/lib64. So how is it even aware of that /usr/pgsql-9.6 version?
Since at least 9.3, it's been possible to configure the server to create more than one socket file (see unix_socket_directories). Recommended practice when you're dealing with multiple client builds is to create sockets in both places.
Yep that sounds good for later, although can't really help me in this case. I guess I can set unix_socket_directory in 9.2 to use the new location but that requires a restart anyway, not ideal.
Would creating a symlink be a workaround? Would that cause issues with server restarts?
Mind that this is only to prepare for the upgrade. We aren't planning to run both 9.2 and 9.6, and eventually 9.2 would be removed.
