Unfortunately there is not a way to cancel the sustaining gifts in mass or change which payment processor account that the gifts are processed in. They will either need to be cancelled individually on the admin side, or the constituents can cancel it themselves.

If you decide to go with the latter, then you'll need to identify these constituents by running a query (BB746184) and send them an email advising that their sustaining gift was cancelled, or will be cancelled and provide a link to the new donation form that is tethered to the new processor and encourage them to recreate their sustaining gifts. If you would prefer not to lose money over cancelling the account, then you can archive the Donation Form, or change the Merchant Account of the Donation Campaign and just leave the payment processor account open to continue processing the legacy sustaining gifts. When the Merchant Account of the Donation Campaign is changed, it will not retroactively affect existing gifts. Any new gifts processed in the donation forms within that campaign will process under the new merchant account and payment processor.