What are our obligations to our certificants if we decide to sunset a credential? Would the holders of a defunct certification effectively not hold that certification anymore (perhaps effective on their recertification date)? Or are we obligated to keep a recertification plan in place?

You are only obligated to maintain a method of verification for the credential while they are certified.

You should notify certificants when the credential will be no longer offered, and probably advise candidates that the credential will not be offered after a specific date.

As a courtesy, for two years after the last credential has been terminated, you may want to make available online a list of certificants and the last dates they were certified.  E.g., Jane Smith, ID 38747, – Held ABC credential from 2/2010 – 5/2017.  This would mitigate the decline in value resulting from the termination of the certification.

As far as I know, there are no legal guidelines for sunsetting a credential.