I spoke to one of the departments that implements billing and promotions about a week ago regarding how the MHS/MBBC usage works after the promo ends, and they told me directly that the update will remove the free feature, and then at an undisclosed amount of time later, they will start randomly auditing accounts where the phone does not show the updated software version to check for MHS/MBBC usage. Out of pure courtesy, they will likely send some kind of cease and desist notice, but to be honest they are not require to do any such thing... especially when an audit shows the software push was rejected, interrupted, or in other ways manipulated. Believe it or not, they did plan for the contingency of people refusing the update.
Your standard Care/Tech rep will not know this until the first billing adjustment is issued and the customer calls in furious, sadly.