[eluser]RaZoR LeGaCy[/eluser]
I don't think it is as easy as you think. I would like to migrate from freakauth which was one of the first to tank_auth or another new auth system. I have looked into the code and it may be a little difficult.
The encryption methods and the tables/fields make it harder to simply upgrade.
Maybe time to bite the bullet and freelance this job.