Note that to do it the right way you should do this in single user mode, which means while you're at the data center, and will have to shut down your OS and bring it up in single user mode.
If you don't want to do it that way you can shut down exim, pop and inetd, before you do the following:
# cd /var
# mv spool /usr
#ln -s /usr/spool spool
If you've turned off those three services, it's now time to turn them back on.
If you've switched to single-user mode it's now time to switch back to (regular) multi-user mode.
Note that if you're using the quota system this will probably break it.
The above is from memory. It is NOT tested. I take no responsibility for breaking your server.
Jeff