Good question - and depends on the main Registrar.
Resellers have to sign "agreements" but registrars also have legal issues to mind if a reseller goes under. Just think what the loss of 1,000s of domains would do to ANY registrars reputation!
With BulkRegistrar another reseller is assigned the domains, but it takes up to 2 months sometimes - and needs a lot of complaints from customers of the "old" reseller. It's not a daily accurance - but does happen.
StarGate - their API is so hard to install and requires Windows / NT servers, so not much reseller stay with them. I don't see any reference to this matter in their docs.
If I was really interested - I'd just email sales at each registrar. I'll see what eNom has to say about this today, unless someone knows already?