I liked the idea of up-front charge so I switched to it on the 1st of this month, however I think it is not really fair to the patrons who sign up on 30 August to be charged again 2 days later.
I have two suggestions:
Make the up-front monthly payment proportional (or at least allow the Patron to decide) so if someone signs up on 30 August they will pay 2/31 of the money for that month and then the regular payment on the 1st of September.
For Patrons who signed up in the second half of the month (and were charged immediately) skip the next charge. Maybe even allow the creator to decide from which day in the month this happens.
In either case the creator should probably need to opt-in to the feature and in either case you might offer the patron to have full payment both when signing up and next month. After all many are there to provide support and not to get the benefits.