I do not believe there is an upgrade path for SI Drums.
SI Drums are GM MIDI Drums so any GM MIDI Drum loops should work fine.
To integrate new MIDI loops into the SI Drums loop browser locate the Patterns directory. In my case, running Win7, they are in "C:\ProgramData\Cakewalk\Studio Instruments\SI-Drum Kit\Patterns". Then create a directory for your new MIDI loops and add the loops to the new directory. The next time SI Drums is open, press F5 in the loop browser and the new directory should show up. NOTE: ProgramData is a hidden directory.