It's a big mess: transferring a domain from module-1 to module-2 requires transferring it from Registrar-1 to Registrar-2 ... and it's not so easy, because (depending on domain extension) you need to follow all the required transferring steps for that particular domain extension.
Even more: you can't (or you couldn't) do it without the registrant's approval.
And, before going on with the transfer, you should warn your user about it, because he'll get various email messages about his domain being transferred to another registrar...
So, it's not impossible: we did it for a few thousand domains, to leave a registrar we wouldn't use anymore, but to do it we realized a custom script, sent a lot of messages to users and registrants before doing it, we needed to monitor every transfer process strictly, got some errors that required to be manually managed...
More than a big mess, a real nightmare.