Pulled the trigger. 57 minutes of downtime. Up and running strong. No issues. I'm back in a supported environment and can send PIN messages from the BES again!
My rollback contingency plans are sitting unused, what a heart-breaker...
BES 220.127.116.11 5/19/04 until 8/31/10 RETIRED
BES 18.104.22.168/Domino 6.5.4 FP2 580 users RETIRED
BES 5.0.1/Domino 8.5.1 223 users RETIRED