Quick and Easy Answer: NO, an end user nor the factory can change the frequancy band of a transmitter or receiver.
How I know this and other info on Senheiser wireless:
I run 12 Senheiser G2 in Band A, Was given a Band C receiver from elsewhere in our School District. Had same question. However, Senheiser has put enough
practical thinking into their product to make them backwards compatible. Meaning a G3 transmitter will work with a G1 receiver, vis versa. The huge item you need to pay attention to is the frequency band. Something I discovered is the new G3 equipment has a frequency
shift. If you set using 'Bank,
Channel' the frequencies will not match. The simple work around is using the tune function.