FreeBSD upgrade on production server

[SUPPORT]

Hello everybody,

I wish to upgrade my production firewall / mailserver / DNS server from
4.3 to 4.8. The simplest way seems to be use of CVSUP. OK, but...

Is it safe ? What should I backup ? There is running well-configured
sendmail - are there some changes in its configuration between
versions 8.11.3 used in FreeBSD 4.3 and 8.12.8p1 used in
FreeBSD 4.8.
This is my only mailserver and I don't have an secondary
if something fails...

Please, advice if you have some know-how :-)))

Peter Rosa