I must be missing something - why not just hash all the plaintext passwords, store the hases on the new version of your site, and have it so when users login to the new site it does the right thing.
Why should the users have to know anything?
Why should the users have to know anything?
It's just important to not lose any of the new users which this industry has a high churn rate (and we are providing the new service that they have previously been paying for for free, but we have a lot of upsells)