there ain't none, however there are a set of big boys rules....
when taking other peoples tat over you either have to the product knowledge of how to over come any lock or quirks of the product
or have enough £ in the job to put your own stuff in from either the start or retain the subscriber for enough years to make it pay..
confiture demain, as you might say ?
Sadly a few hundred quid for new control can be a problem as the subscribers can be living had to month with the latest I-Phones, Range Rovers & shirt sponsoring the local professional sports team... taking priority