Help-Limited Exercise

I have been going to a kickboxing class and I really enjoy it, however my knees don't love it!! My knees are awful!! The doctor has already told me that I will need a knee replacement but because of my age I should wait because I will need another one if I get one now!!! Can someone give me some suggestions as to what exercise I could do? I don't want to gain the weight back I've lost!!! I know I will continue to make good decisions with my food choices.