I'm not sure it'd be worth the effort to change from 7.50r16 to 235/85r16, they are almost the same size. Here's an old shot of my two (well, three), the Defender wearing 235/85r16 BFG KM2 Mud Terrains for comparison.
http://i940.photobucket.com/albums/a...psc4607efd.jpg
The 235s are a little wider, but diameter is the same.
255s are generally accepted as ok on 6.5" rims. For what you want, I would just run a single set of 255/85r16s all the time. Two sets of wheels with slightly different sized tyres is a waste, for all the time and effort it takes to switch them over you'll just leave one set on. Trust me. Maybe I'm just lazy :p