We have seen numerous people making a mistake or misunderstand the plan details and get stuck on the plan they didn't want. This happens because a plan change would cause them to lose the money they have already paid, even if it has been just a day.
I understand that this is a prepaid service and there are no refunds. Thats fine. However, for new customers who are likely to make an honest mistake, it would make a lot of people happy if there was a grace period of anywhere between 2-7 days where the new customer can make 1 plan change and only pay the difference (or receive credit in the account for future use if downgrading the plan).