I really wouldn't recommend leveling as holy. Given your exp as a pally, you can get away with off spec healing. Depending on when you cam into TBC, you may or may not be familiar with down ranking spells, as an offspec healer you are going to have to master this to stay mana'd up. Raiding they are a necessity tank healer, once you get enough MP5, you will be the sludge that keep your tank alive.