Don't worry, you don't lose your promo plan unless you either make changes to it yourself, or you let it expire and fail to reactivate it within 90 days.
If today is your payment due date (as listed in your account overview page), then here's what to expect:
1. Expiration of completed period. Account will show "plan expired" on payment due date.
2. Conversion of all rewards to available funds at midnight; rewards reset. Payment is not collected yet at this point. Account will now show "Your account has been suspended" in big red letters.
3. In the early hours of the morning, available funds will be used (if any), credit card will be charged (if necessary), and plan will be renewed. Account status will then show as "active."
Throughout this process, your service should be uninterrupted. You only need to contact a moderator for help if your service actually stops (can't make calls or use data).
