Many old bathroom pull switches have screw on lids. Over the years they get painted over and become stuck fast.
You could fiddle around finding the gap and cleaning out all the age hardened layers of paint - OR - given that you're intending on replacing it - isolate the circuit and break it with a few light blows from a hammer - best to go upwards with the blows rather than sideways to avoid too much ceiling damage.
Peccavi, February 2009
