Be aware that many of the new digital systems (such as QLXD and ULXD) are not compatible with many mics, especially inexpensive models. Their RF modulation schemes use variants of AM transmission that "leaks" into the audio signal through their cables and mic elements and sounds awful. Make sure to test this thoroughly with your existing mics before you buy or rent a ton of channels.
Do you strictly mean like handheld mics fo other brands? Our Sennheiser handheld mics currently are all on the B band except one so would also need replaced as well. The majority we use are micro lavaliers - Countryman and Microphone Madness brand. There wouldn't be an issue with the microphone element itself going into the Shure transmitter, correct?