Do you have (and are willing to share) the credit card information on file for each customer? Or is the new Sortfolio owner expected to contact or otherwise prompt existing customers to re-enter their payment information?

It is possible 37signals doesn't have the information, but the gateway does and would be willing to transfer the credit card numbers. Either way, this is critical detail that needs to be worked out.

I don't see how this could be done if they are PCI compliant.

there are several ways to reach compliance; not having the information is just the easiest