advice on upgrading production FreeBSD 5.4

[pobox at verysmall.org]

I inherited a production FreeBSD 5.4 used as a web/mail server (Apache, 
PostgreSQL, php, qmail, vpopmail, Courier). Could anybody help me with 
information about a web resource on upgrading such system (all, OS only 
or component by component) with minimum downtime. I looked into cvsup 
and portupgrade - is this the right way for production systems or is 
there another one? For example is it possible to have the older version 
running until the new one downloads/compiles and then to replace it 
within seconds? Also - what if the new version does not work correctly - 
is it possible to keep the old and revert to it.

Thank you,
Iv