I don't think you should be able to convert Solo gear into Raid gear or vice-versa, you should have to earn each (although storing extra sets and swapping between them should be convenient and effortless). The way I see it, the base stats of each should be completely identical, for the same tier of gear. The distinction should be that Raid bosses should be highly resistant to normal damage, but weak against a special damage type, and they should deal a unique damage type that largely bypasses normal armor. This would make them very difficult to fight using traditional armor and weapons. Raid armor would of course reduce the special damage type, and raid weapons would convert a portion of your damage into the appropriate type. The higher the tier, the more protection/conversion occurs (and of course the more the next level content would demand). This would mean that the very best raid gear would be equal to, but not any stronger than, gear available through solo play, and it would definitely be superior while inside any raids. I don't think it's strictly necessary that solo gear be better than raid gear under any circumstances, so long as it's at least equal.