I just tried this, and found out there are some other issues:
1) Only do this if your DotNetPortal is a single portal installation. and only this installation uses the connected database Otherwise you have to modify the following SQL Scripts in a way that only users of this portal are affected! Don't blame me if something goes wrong, so try it in a test environment with a backup of your installation and database!
2) You have to update the Users table with the following SQL Script:
SET Username = Email
WHERE Username <> 'host'
(You may forget the last line, but then the 'host' user has to login with his email as well. Use the username of the host, if it's not 'host').
3) You have to update the aspnet_Users table with the following SQL script:
SET UserName = (SELECT Email FROM aspnet_Membership WHERE UserId = aspnet_Users.UserId),
LoweredUserName = (SELECT LoweredEmail FROM aspnet_Membership WHERE UserId = aspnet_Users.UserId)
WHERE LoweredUserName <> 'host'
(Same as above!)
4) After this, recycle your AppPool.