Yes it is possible. here's some info i found on it.
Version Upgrade (from SBS 2003 Standard Edition to SBS 2003 R2 Standard Edition)
Version Upgrade* (from SBS versions 4.0, 4.5, 2000, or 2003 Premium to Windows Small Business Server 2003 R2 Premium Edition)
Product Upgrade (from SBS 2003 R2 Standard to SBS 2003 R2 Premium)
So this is confusing. i think we need to T72-01415 and T75-01260. I'm in the same situation as you but instead if ISA i need SQL 2005
here is the link. Hopefully someone from MS will confirm this for us.
Anyone feel free to jump in and answer this question. Would it make more sense to Upgrade a SBS server let same to Windows 2003 r2 and migrate Objects to a new Forest or to install an new Windows 2003 r2 server move the FSMO and Global Cat to the new Windows 2003 re server, then migrate to new Forest?