I have an Ovation Celebrity.
It is about half a year old. I had it set up at guitar center, and the buzzing was fixed for a while.
Now, it has come back.
He set the action real high, and I would like it lowered, while stopping the buzzing.

It has Elixer 12's on it now, but have a pack of 11s too.

It only buzzes from the 12-15th frets, only on the wound strings.
92% of teens have moved to rap. I am one of the 8% that listens to real music.
Could be a couple things. If the bridge fell out changing strings it would be lower and causing the problem.

If the neck has a bow then the truss rod needs adjustment and that could cause the problem. If you went to a lighter gauge of strings that can cause the problem as well.

Take it to a guitar tech and have it adjusted.
Definitely bring it to a tech and tell him or her you want the action lowered and the buzz gone. It could be several things...uneven frets, a trussrod adjustment, a bridge adjustment, or even a nut adjustment. As long as there is no major warping the tech should be able to improve the action and eliminate the buzzing.