1. Can I shut down the Exchange SBE BES and use the licensing for the new Evaluation Exchange BES?
2. Can I (someday) move the licenses from the Groupwise BES to an Exchange BES?
Licenses are not platform specific. You can use a GW license on a Exch BES and a Domino license on a GW BES. It doesn't matter.
3. Whichever path I end up taking, would I be able to license the evaluation BES without having to redo enterprise activation on the BB's that are already set up on it?
The users won't be affected.
If this is truly an Evaluation BES it will eventually expire, why go through the trouble of having to reinstall the BES again? Evaluation BES comes with evaluation licenses as well. Using an Evaluation BES in a production environment is more work if you ask me, if you have the licenses and the software available. You can use the same license on 3 different BES if you like. The licenses are not checked against any RIM database or anything.
If this BES is not an actual Evaluation copy then forget what I just said