I also tried to change my payment info, spent an hour being sent in circles trying to follow the instructions given at the link below as well as other help forums, decided it was futile to change my payment info. Tried to cancel my subscription with the same results. It is obvious they get enough people giving up to make this worthwhile to them, so...
I have now documented my attempts to cancel, will simply challenge the charge when it comes use this documentation to prove my point. I will also share this experience on social media since the only way to avoid this hastle is to NOT USE THIS SERVICE IN THE FIRST PLACE!