How big of a pain in the xxx is this project? Is it a easy thing to do or a multi-day nightmare?
Unfortunately, based on the information you provided it is impossible to say how long this will take. For example, I don't know how many mailboxes need to be moved, the size of the mailboxes to be moved, or the speed of the network over which the data will be moved.
Generally speaking, though, moving mailboxes from one Exchange 5.5 server to another is a fairly easy task. Note, though, that if the old Exchange server was the first server in the site, you'll want to follow the procedures here before you remove it.
Dig deeper on Microsoft Exchange Server Mailbox Management
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