Is there an echo in here?
I hear this all the time: 11r24.5 are cheaper and easier to find than 12r22.5. Both true, however, I have studied the cost difference and the availability issue, and, frankly, most of us will not save enough over the lifetime of our buses to justify the expense of changing the wheels.
The short answer is, yes, you can change wheels. Make sure you get the correct mounting (hub-pilot versus stud-pilot aka "Budd"), correct hub bore, and correct bolt holes for your application. Then, make sure you recalibrate your speedometer, and, if DDEC-equipped, the DDEC RPM parameter. Lastly, measure your wheel-by-wheel weights to get the correct load range and inflation for the new tires.
Remember that the 24.5 wheels will give a stiffer ride than the 22.5 wheels under all conditions.
I am not sure why you would want to run different size wheels on the steer axle vs. the rears, although this is common on very heavy coaches.