i'm thinking more bridge a trixbox/asterixnow/freepbx (something asterix based) to a BCM and slowly migrate users so i don't have to buy 200 handsets all at once and re-train 500 users monday morning.
BCM400 for voip isn't looking great looks like you need a license we don't have and @john 's comments aren't promising. I'm certainly not upgrading to another BCM, the idea is the (pick 2 not 3 ) cheapest, easiest, least hassle migration path.