Upgrade/install over earlier version

[Dr Robert Young]

Does anyone have information on the installation/upgrade of V3 of 
Spamassassin, on a system already running  V2?  Should the new version 
go "on top" of the older one, or as a separate product install? Any 
issues one should be aware of?

I am installing on RedHat 6.2 and using a fairly recent version (last 2 
yrs) of sendmail (I'll have to look it up for the precise version if 
that matters).