I'm going to have to do the same on my partner's car soon, the front discs have been on for 70,000 miles and are close to minimum thickness. It'll probably require penetrating oil and a few whacks with a lump hammer. The centre hole in the disc has to be a very close fit to the hub, so it doesn't take much corrosion between those mating edges to make the disc stick. When tapping it from behind, rotate the disc to try and keep it square to the hub as it starts to budge.
I once owned a 1996 Mondeo and the front discs had seized on so tight that I had to give up and take it to a garage for them to change the discs and pads.