Not really an answer, only my 0.02 cents.
I think that you will spend as much time participating in the MembershipProvider tournament in the new Identity as it takes to properly upgrade the new Idenitty infrastructure.
I made an update on two different systems, as not small (one 200K, another 70K lines of code) with a large number of users. The smaller system took me 7 man-days, more than 5 days (I knew that I was doing the second time). Both systems had an extended amount of user management code; one of the systems contributed to the impersonation of the other user to administrators. Everything worked smoothly and there were no downtime. Users did not notice the difference.
But after updating things using user management / authentication were much easier, you will get 5 days spent on updating in the shortest possible time. Think of it as investing -))
I looked at the source code (in the decompiler) of the MembershipProvider, and many things are static, dirty, sealed and just unmanageable. I would say that it would be easier to flush it instead of creating older code, just to save the dying library.
In other words, it will be easier to update everything, rather than trying to reuse old files.
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