The answer depends on how you handle your resold domains.
If you use something like Registry Rocket the domains stay in your own account, so in the event of an account move you could simply push the domains as has been mentioned before.
However if you use PDQ or anything else that results in your customers' domains residing in sub-retail accounts, you would not be able to move them. Sub-accounts cannot be pushed from one login to another. eNom used to move them on occassions in the past, but I believe it was causing data integrity problems.
Best wishes,
Simon.