redhat 4.2 migration to 6.2

I am planning to migrate my linux redhat 4.2 server used primarily as an authentication, mail and webserver, to linux redhat 6.2. However, I am relunctant to do this with the thought of some key issues that might possibly be raised during the process. What are the key issues that I have to guard myself against if I migrate into a higher version of linux redhat.