The churn can be reduced if they make it immediately clear in Self-Serve what the conditions, duration, and limitations will be. As in, some kind of in-your-face message like "changing your plan now will disqualify you from getting the rest of your signup bonus data" or whatever.
And if they perhaps send SMS notifications every month which remind you that you got (or will get) that bonus data by sticking with the plan. Because too many people set up AutoPay and never bother to login Self-Serve again - unless there's a problem.
And if they perhaps send private messages to the linked Community account (if there is one) with the same information. Maybe overkill but automated messages cost nothing.
If PM basically takes away the customer's ability to "accidentally" (unknowingly) abandon the promo conditions, and if PM keeps the customer updated with promo details/status (instead of forcing the customer to remember and do math and stuff, lol) then the only ways left to "lose" the promo data are to intentionally ignore the warnings or to stick around long enough for it to expire. (And I'd bet that few of these customers would run off the moment their promo deal expires if PM just happens to send them a one-time "hope you stay" gift of 2GB just before the last renewal.)
