We moved into an older house, and one of the projects is going to be replacing the 2-prong receptacles and changing them into 3-prong. However, there's many layers of paint around/over the plates, and the screws as well. In the past I would just pry it off with a screwdriver, but I know that's not the best method. What other methods are appropriate?
Use a utility knife with a very sharp new blade and carefully cut around the perimeter of the plate at an angle as close to parallel to the wall as possible. This should get you a nice clean separation of the plate from the wall.
For the screws - if you're buying new plates they'll come with screws, and if they don't, you can find extras (including extra long ones) at your local box store.
Now as Tester pointed out, if you cannot even remove the screws, then you need a thin flathead screwdriver and a tack hammer (or a very light touch with some other hammer).
Set the head of the screwdriver against the head of the screw and lightly tap the handle with the hammer to break off the paint enough so that you can identify the actual slot in the screw. Once you have the slot identified, rotate the screwdriver so that the tip is at an angle set in the slot, and drive the tip of the screwdriver down the slot to dig out the paint.
Once you've cleaned the slot sufficiently, remove the screw as per normal use.