Autopay Delay Problem

My bank account was closed before Xfinity charged for autopay although I did enter a new account for Xfinity to use to withdraw the funds.  Typically ACH is made by month-end, but no payment has shown up on my new account.  Will Xfinity know enough to use the new account if the transaction with my old account was rejected?  How long should I wait for resolution without additional late charges?

Thanks.