Mail Server Controversy

I am currently trying to replace my FreeBSD mail server with a RH8 mail
server. Is it possible to have both mail servers exist together during
this process. I have new clients that need mail service and I want to
know if I can have both systems running concurrently. If so, what are
the MX records I need to enter in order to have them work. Lastly, is
there a program that will transfer my current users from the FreeBSD box
to my RH8 box. Thanks for your help.

Russ