I probably have more issues with users forgetting their user name than any other single issue on the site.

I have tried setting the site to use email as a username, but I just get an error (this is in DNN 07.04.00 (353))

Message:Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

StackTrace:

InnerMessage:Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

InnerStackTrace:

   at System.Data.SqlClient.SqlConnection. (SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryHasMoreRows(Boolean& moreRows)
   at System.Data.SqlClient.SqlDataReader.TryReadInternal(Boolean setTimeout, Boolean& more)
   at System.Data.SqlClient.SqlDataReader.Read()
   at System.Data.SqlClient.SqlCommand.CompleteExecuteScalar(SqlDataReader ds, Boolean returnSqlValue)
   at System.Data.SqlClient.SqlCommand.ExecuteScalar()
   at PetaPoco.Database.ExecuteScalar[T](String sql, Object[] args)
   at DotNetNuke.Data.PetaPoco.PetaPocoHelper.ExecuteScalar[T](String connectionString, CommandType type, String sql, Object[] args)
   at DotNetNuke.Data.DataProvider.GetDuplicateEmailCount(Int32 portalId)
   at DesktopModules.Admin.Portals.SiteSettings.UpdatePortal(Object sender, EventArgs e)

What can I do to migrate my users from using their old login names to using their email addresses?