Your reward in terms of dollars gets applied first against your monthly plan charges and then rest amount will need to be paid by you. Hence your rewards amount will not be payed but you can use it against your plan service.
Ideally, all the rewards are associated with your account hence if you change your mobile number, your account should still be active and same hence no impact will be on any reward.
Ideally, all the rewards are associated with your account hence if you change your mobile number, your account should still be active and same hence no impact will be on any reward.