Only you can join the points program quiet agreeing to it. Switching plans will not change the rewards program you're on. If you have any promotions you change plans.
No you won’t loss anything, your old rewards will remain if you switch plans.
Unless there was a restriction on your old plan. Eg. 2gb for 12 months on $25+ plan, if you switch to the $15 plan. You will loss the 2gb x 12 months bonus.
@immyonlyfan You'll stay on the old rewards and don't lose them as they're attached to your account and plan changes doesn't affect them but are advised to choose Change at Next Renewal option as PM doesn't prorate for unused days left in your current. Also rewards won't be applied if you Change Now but will be available for the next renewal.
