I'm just about to configure Postfix after having tested at work on Redhat 8, we found this link that was helpful (although for Mandrake but still useful):-
As to your specific questions, we didn't actually remove sendmail, just stopped it being loaded as a service at start-up.
As for upgrading, on all Redhat versions we use a combination of Apt-get & Synaptic for managing patches/upgrading, very reliable and makes package upgrading a breeze. AFAIK once you disable sendmail and enable postfix, postfix is your default.
To configure postfix we used the (latest) webmin module, the only options we changed from the defaults are (for fixed ip, inbound port 25 forwarded to server & using a DNS management service) :-
What domain to use in outbound mail - Use Domainname (i.e. actual mail domain equals domain of our server)
What Domains to receive mail for - Whole domain
Send outgoing mail via host - Deliver directly
Network interfaces for receiving mail - All
Root - firstname.lastname@example.org
Destination address for unknown recipients - email@example.com
We got so far and it all appeared to be working ok, however we could not find for certain that we could deny it being used as a relay so we shut it down until after the holidays. Also we intend to do some serious research to look at the config more closely, there is a wide gap between working and working correctly :-)