My company has a central site and two branch offices, all running Exchange Server 5.5. Can I migrate the central site to Exchange 2000 Server without breaking the branch offices' ability to route mail and access public folders?
Yes, if you're careful. The mechanics of planning a migration are too complicated to go into here. However, one relatively simple way to migrate the central server is to add the new Exchange 2000 server to your existing central site, then migrate mailboxes and connectors to it from the existing Exchange 5.5 server. Be careful to set up your Active Directory Connectors (ADCs) properly. If you configure the components correctly, when the new server is on board, mail and public-folder traffic will pass without difficulty.