Exchange 5.5 to Exchange 2003

I am migrating our exchange from Server 2000/Exchange 5.5 to Server 2003/Exchange 2003. I have exchange installed on both machines. They are in the same domain. They are in the same server groups in AD with full privilages to each other. When you look at the exchange manager on both machines, they can see each other but I cannot get them to transfer the existing mailboxes from the 5.5 machine to the 2003 machine. Can anyone help? I have followed all instructions to do this. HELP PLEASE…..