There's no easy way to do what you are describing with a single domain controller. Changing IPs is not a big deal so long as the SRV records are updated in DNS.
Personally, I'd bring up a second server. Promote it to a DC (DNS installation implied), transfer the FSMO roles, and start pointing your clients to it via DHCP, demote original server, and then reimage. You're using SBS so that may not be an option and it assumes you have another license and server available. It's certainly a lot cleaner and less chances of something going wrong.
One question though, if you are small enough that you are using SBS, why the class a network?
Personally, I'd bring up a second server. Promote it to a DC (DNS installation implied), transfer the FSMO roles, and start pointing your clients to it via DHCP, demote original server, and then reimage. You're using SBS so that may not be an option and it assumes you have another license and server available. It's certainly a lot cleaner and less chances of something going wrong.
One question though, if you are small enough that you are using SBS, why the class a network?
