I could see that the "Add a New Rule" feature would be a convenience, but there is a quick and easy method to do what you want now without losing an existing subscription.
Just go to Standard View>Television>All Programs, pick any program
in the future, and then record button becomes available, click it, and set up any recording subscription you want.
The only problem with that is that the subscription retains the original program name, which is annoying, and is why I have asked for that field to be editable as well, in addition to having the subscription Rule Name editable. The Rule Name is the field used for sorting and display in the Recording Rules list, but it would be nice to create a rule with the correct Program Name, and a configurable, meaningful Rule Name. I would use this to set up rules for programs that aren't in the current EPG, but I expect to appear in the future, and I want to capture without having to remember to create a rule when they are first scheduled. Thus, I could avoid missing the first episodes if I didn't see that the program had started to be broadcast.
I have also asked for additional fields to be available for rules, so I add my voice to that request.
Of course, all of this relies on quality EPG data, so we will need to take into account the possibility of missing data when setting up rules.