How many users are you talking for this site? Is it publicly accessible, or something used within a company LAN?
Transferring your existing membership database to the schema generated by the MySQL provider could get interesting. I'm not even sure how similar the table schemas they use are, though I would hope they are quite close.
If the number of users is relatively small, I would strongly consider forcing a password reset upon changing to the new system.
edit just saw you added postgres while I was typing - if your database choice is still up in the air it may be easier to use an ORM-based provider that offers you a bit of portability (and consistent behavior)
if using nhibernate, I was recently looking for an nhibernate-based membership provider and ended up choosing this one: Fluent NHibernate Membership Provider (CodeProject). Got it to work for me with minimal modification, but its for a greenfield project without these concerns. It is on mono though :)
found this: Membership Providers - see the "DataSchema" section, aspnet_Membership table. Looks like getting the salt won't be a problem, so you should be able to get this working w/o password resets.