Hi Jason,
Before you do anything make sure you have an updated backup of the System State (Active Directory) and the Exchange Logs. You will want to do a full backup seperate to this as well. This will ensure no data is lost.
Another good option that I use myself is to take an aimage of the SBS Server using Symantec Ghost, This allows a quick full recovery option if things go very wrong.
It is very Ideal to migrate to another system, you can pickup a second hand dual processor 3GHz server for under $2000 these days and this can be used as a fail over system. I would highly recomend that you get a product like VMWare and take a virtual image of the server using PXE or another method. This way you can test the upgrade in a Virtual Environment.
I have done a few Server 2000 to Server 2003 Upgrades without a second server and I have learnt not to be surprised if services simply stop working after you updrade. The more preperation and testing that you do the better things will work out.
Best Advice I can Offer - Ghost this server so you can roll back quickly if you need to.
There should not be too many issues as the root system is the same, but make sure you have good backups.
Regards
Jarad C
