All electrical fixtures are required to be grounded by the National electrical code. Grounding is necessary for a few reasons and should be done for your safety. In some older houses with the wires having a braided material type of cloth over it, there was no ground wire as far as i know. Later on they realized, a ground wire should be available and now they are in all curreant wire products. The ground wire is usually a bare copper conductor used to connect all fixtures to. In the armored cable, (BX) the cable connection WAS previously used as a ground, but that too now has a bare copper wire in it too. If you do have the old wire, as i previously described and it is without a bare additional grounding wire, and you cannot get a ground wire in the box, it should be ok to just change one switch for another. It is NOT the current way of doing things, and I would not feel comfortable without it, but know that it WAS the way to do things many years (decades) ago. For legal reasons, I must advise you to consult a licensed electrician.