Not sure I'm completely understanding the question but I'm going to make some assumptions.
I'm presuming users logon via forms authentication, and that determining the users database version is trivial?
If so then you can simply host your old and new version in separate virtual directories
then make sure they can share authentication cookies. Once the user is successfully verified ( presumably on the new version of the site) if they should be using the old site you redirect them to the default page of that site instead of the new version. I believe the mehod to do the redirect in is RedirectFromLogonPage()
In addition to prevent the user from using a bookmark to go to the wrong site you could put something in the Session_Start()
of Global.asax
that does a lookup of the base URL the user should be using and redirect their request appropriately, you'd have to do that in both the new and OLD versions of the site though.