How can I remove a stuck overhead light fixture?
Looking at the geometry, it can only be paint holding it up.  I would stay away from any solvents.  

I would add a second putty knife, with a stiff blade.  Use a slightly larger (wider to distribute the force of the lifting blade) putty knife against the ceiling (and under the lip of the fixture).  Put the stiffer, skinny blade between the wide blade and the fixture.  Pry up slightly, all around the   circumference of the fixture.  You will break it loose.  

The prybar is overkill (in the photo), but illustrates how to protect the underlying drywall.  (Guess what happens if you use the prybar alone. Ask me why I know :-O)



Good plan by Christos regarding leaving the new fixture, though I believe you will succeed removing the original.