Source upgrade from 6.3-PRERELEASE to 7-STABLE

[Joshua Isom]

I've looked back at a lot of the emails about upgrading a system from 6 
to 7, but all seem to contain some caveat or other, generally ports.  I 
want a "clean" system, but don't want to deal with backing up, 
reformatting, and installing, not to mention reconfiguring everything.  
Since by now many people have done it, what's the best way to upgrade 
from 6.3 to 7.0 for the entire system, base and ports?