I realize you've already spent a fair amount amount of money but purchasing the technician kit at sbmigration.com would be a great way to go

You keep the domain intact, don't need much as your interim server, lots of pluses, not many minuses

Or yes...you could export your mailboxes using Exmerge as well as all your data and start over...lots of downtime for the business.

 I am currently running SBS2K on a PowerEdge 2600 Server with 5 hot-
 swapable 36GB SCSI drives in a backplane with a RAID 5 Config. It is
 time to upgrade.

 The server has 2 decent Xeons and 4 GB RAM so I did not think it would
 be worthwhile to buy a new server.

 I purchased the SBS2K3 R2 software and client upgrades along with
 6-146 GB SCSI drives.

 I would like to know the easiest way to install the six new drives and
 the software as well as migrating Exchange personal mailboxes.

 I only have 20 users, so recreating user accounts is no big deal.

 Can I simply dump the mailboxes onto an external drive and import them
 into SBS2K3?

 If I run into problems, can I simply plug the original 5 drives into
 their original slots and be up and running again?

 Thanks for any help.

You could also buy an external USB HD and a bare metal recovery package like
Backup EXEC system recovery.  Use BESR to create a backup to the attached USB
HD.  Pull out the old drives, put in the new drives, create your RAID,
restore the backup partition by partition remembering to resize each
partition since you've got a bigger HD.

I've used this to move SBS2003 to a new server and it works.

If it doesn't work, just put the old hard drives back in the proper slots.