What you should do is that since you have only 5 mailboxes, you shoud go to each of those mailboxes on the workstations, export the information to a personal PST file to the workstation. Get your new server with exchange ready to go. You should be using the SAME domain name on the new one, make sure it is NOT connected to the same network. In the evening you should then shut down your current server, put your new one on the network, connect a workstation to the new server, attach to the new exchange and send email OUT to a external address such as a yahoo account. If you recieve that mail externally, GREAT. your conncector in exchange is working. You won't be able to receive until you change the IP address on the new server to IP address your old exchange is using. or you can call your ISP and have them change the MX records to that new address. Are you using public or private numbers. This will work if you are using public numbers, this will NOT work with private numbers. You'll have to change access on your firewall. I would just change the IP numbers from the old to new server.
I had 30 workstations I converted over to exchange 2000. I made sure I did NOT do a migration, I created a new exhchnge server 2000 and then imported those PST files from the workstation to the NEW exchange server mailboxes. Worked GREAT. You may get some people REPLYING to old email generated from the OLD exchange server a "undeliverable", Even if the address IS correct, the problem with that is, There is special UID code for that person, its looking for the UID code from the old server. This will soon phase out. All new mail works GREAT!!!!!
I'm really comfortable with this kind of work. Email me if you need some help.
jkennedy@oberlinonline.com