At work, we frequently migrate content databases from one AD (our prod environment) to a completely different one with a different domain (our dev environments). I've observed that the users from the previous AD still remain in the site collection (with their entries in permission lists intact), and you can still view their details as imported from the previous AD. But you probably can't do much with those 'dead' users. I suspect that if I pushed the content db back to prod again, those users would be 'resuscitated'.
I haven't ever done what you're thinking of doing, so I can only really speculate as to what might happen. The ideal outcome is that SharePoint will automatically wire up the existing SP users to profiles in your new AD, based on display name. However, I suspect SP identifies users by their SIDs or something equally specific which will change between ADs. In this case, it probably won't just work straight away, and you might have to use STSADM migrateuser.