register plus no longer works with WP 3.0 and has been replaced by register plus redux. I haven't found anything about upgrading to that plugin and keeping all the data, does anybody have any experience with that?
I didn't get an answer here, but I did migrate my site to regi-plus-redux. I did have to recreate all of the custom fields, assuming the custom fields were recreated w/ the exact same name and options they did show up for the users after the migration.
It wasn't a difficult migration, and anyone using regi-plus who wants to upgrade to 2.7+ will need to make the migration, don't worry it won't cause you problems.