I switched to Holy at 64 from leveling Shadow all the way. When I hit Outlands, I focussed on healing gear, knowing I was going to be switching at some point. I main healed Ramparts, Blood Furnace and Underbog several times as a Shadow Priest, so it can be done.
*edited for spelling*
Edited, May 22nd 2008 7:44am by Zatire