As an amateur radio operator, you can do whatever you want to the gear as long as it complies with FCC regs when you're done. That means it has to be legal for YOUR license class. You cannot make changes to gear for operation in the 72MHz band but you can make changes if you end up operating in the 50Mhz band (if your license class allows use of that band). Hams often build their own gear so modifying someone else's isn't a legal issue. I was a ham long ago (Advanced class) but was silly enough to let my license lapse while I was in college.

As was stated earlier, this isn't a trivial change. Moving from 72Mhz to 50Mhz is a major frequency shift and will require fairly extensive modifications to the RF circuits (not just a crystal change). Your switching from roughly 4m to a 6m wavelength. Just like with any transmitter or receiver, you're going to have to retune oscillator and amp sections to get them working on the new band. If you are comfortable with this then go for it.

Mark