Best way to migrate Exchange 2003 in a particular situation

A client of mine is running on old hardware that is going underwater with Exchange 2003 (ungodly slow RPC and disk I/O processes). I am bringing in a new server and plan to install SK3 on it (cannot do 2k7). Here is where it gets interesting with some challenges:
1) Old server is running OWA and RPC over HTTP
2) another DC is running BB server pointing to old Exchange server. I found a document on how to migrate that process to the new server.
3) Client was recently bought out by Fortune XXX company with a VPN tunnel between us. We have to do the "mother may I" approach of having specific ports open to specific IP addresses which take a week+ to implement. This is done for the client's users who might be at the "corporate" office.

Knowing this, which of the following would be best to do, AND anything specific to know when proceeding?

1) New server gets its own name and IP address. Migrate mailboxes over. Old box remains as bridgehead and does OWA, RPC over HTTP, and BB. No changes need to be done on firewall to point to new ports for the Fortune XXX side. Concerns are that the old box will fail at some point, however, company is moving within 6 months so can retire it at that point and worry about the rest of the migration at end of move.

2) New server goes on network with its own IP and new name. Migrate services and mailboxes over to it completely. When done, decommission old one, and then replace the new one in old one's IP address. Firewall changes should not need to be made?

3) (CIOs suggestion). Backup old server completely. Install new server with name of old computer's name and IP address. Restore Mailbox store and setup components on it. My concern is the whole decommissioning process, and, even though it has the same name, it has a different SID, creating a whole new can of worms.

4) Open to other possibilites..