As I have a very old server (which serves my purposes) the newest version of webmin won’t run on its OS. I am going to try to move back to an older version. Namely, back to 1991 which did work. If I try to install 1991, when 1994 was already installed will there be a problem? Do I have to remove the installed newer version or will the installation of the 1991 version take care of those things?
1.994 works on CentOS 6, as far as I know. There were some bugs in restart behavior (new systemd stuff) that requires killing Webmin and restarting it (or just rebooting the system) on some systems.
Edit: But, also, you should not be running an EOL distro.