I've been talking to RIM and Dell, and both are pointing me to the same solution, so I thought I'd post it here...
We were originally licensed for the SBE, later "upgraded" to BPS (freely), and up to today I'd only heard of the BPS to BES upgrade license for $2800 (with a little uncertainty whether that covered already licensed users or not). Now I hear (and have an official quote) that RIM is allowing SBE licensees to purchase a 1yr support plan with which you can request a free Upgrade Key covering both the server software and up to 30 active users (BPS' max user count). The 1yr support plan cost is $1299. I had a 3-way call with Dell and RIM to confirm just this morning. Dell product is listed as: BES-ADVANTAGE PROGRAM INCLUDES 1-30 ACTIVE CALS LICENSE (A2819346)