We are in the process of migrating from GroupWise mail to Exchange and currently use a BES for our backend.

What are our Blackberry Server options as we move forward?

FLHJR

There's no migration path for Groupwise to Exchange as far as BES goes.
You need to purchase BES for Exchange, set it up as a brand new server, wipe all of your handhelds and then activate them on the new BES.