Yes, usually you can - going from 4.3 to a later 4.x is fine using yum upgrade (or just updating yum update) but going from from 4.x to 5.x or something like that is out of the question and isn't possible.
You need to make a backup of /usr/sbin/suexec which you must copy back if httpd gets updated. This is because suexec gets overwritten when httpd gets updated, and this will stop all your cgi scripts from running.
You should also make a copy of /etc/named.conf and /etc/php.ini, along with /etc/sysconfig/named if it exists. These are unlikely to be needed, but it is worth doing just in case.