Is it feasable to switch 'class' styles halfway through the game?

I play a level 25 nord, generally using two-handed, archery and light armor. At this point I'd like to try playing a pure mage, but continue on where I am at in the story. Do you think, with enemy level scaling in mind, its possible to change over while still staying at expert/master?

Most of my increases have been assigned to health, with some to energy. I have no mage perks currently.