It is not possible. Changing the number is an irrevocable feature.
Reserving a number and using on the 2ndLine app requires certain infrastructure changes and setup. Similarly, changing a number also requires a procedure to remove the number from 2ndLine as well as setting up a new number. Hence there is no possibility to undo once the number is changed.
Pro Tip to setup uninterrupted Subscription: Balance funds on your Apple account that works like a reserve, and use Auto-topup. Apple will deduct subscription amount first from the fund and then from your configured payment method. Here is a guide from Apple to help you to add fund to your Apple Account.