We recently feature toggled exactly your use case - switching to a new API from our payment provider which needs more information.

You seem to be working on the assumption that everything has to be toggled or nothing. What we did was to collect the additional information for everyone (its a low risk change after all), and then slowly roll out the new payment API, starting with staff, then friends & family, and finally doing a phased rollout to customers.

Feature toggling not only allowed us to roll it out, but also to rapidly roll it back to deal with issues, and resulted in a very smooth implementation of a feature that had potential to directly affect revenue.