New Exchange Server

I am setting up a new Exchange 2000 server and slowly migrating users from our current Exchange 5.5 server. In order for users to be able to receive messages at the new server, do I need to create an additional MX record with the IP address of the new Exchange 2000 server? Will this conflict in any way the ability for users still on the Exchange 5.5 server to receive mail?

Thanks for your time.