There are all sorts of reasons for replacing tires that still hold air. Tread gets worn out, they might start to bulge in some places and they might be severely flat-spotted. Eventually, every aircraft owner will be faced with tire swaps.
There’s no reason why you can’t tackle the job yourself because it’s one of those DIY maintenance items that the owner of the aircraft can complete and sign off. But there is some critical disassembly and reassembly required. Here’s a primer to get you started.