If you need needles or a new cartridge, the folks at Canadian Astatic (I have no connection, just have bought from them) are very helpful. You can send the model number of your set and all the numbers off the cartridge and they can match it from their vast store of information and parts. Upgrading to a magnetic (with preamp) might be possible, but depending on the capabilities of the changer, tracking at as low a force that you'd like might not work. Changers generally take more tracking force than manual turntables because they are dragging around all of that mechanical baggage to operate the changing mechanism.
http://www.canadianastatic.com/