OK, this is what I made. It also updates the tables in Active Forums. Guess I need to do that also in some other tables like Blog Comments and NewsArticles. Is this method correct?

DECLARE @OldUsername varchar(50)
DECLARE @NewUsername varchar(50)
SET @OldUsername='enter old username here'
SET @NewUsername='enter new username here'

BEGIN TRAN

 update aspnet_users
 set UserName = @NewUsername, LoweredUsername=lower(@NewUsername)
 where lower(UserName)=lower(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in aspnet_users table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in aspnet_users table.'

 update users
 set Username = @NewUsername
 where lower(UserName)=lower(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in users table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in users table.'

update NTForums_Posts
 set Username = @NewUsername
 where (UserName)=(@OldUsername)
 IF @@ROWCOUNT=0
  PRINT 'User ' + @OldUsername + ' not found in NTForums_Posts table.'
 ELSE
  PRINT 'User ' + @OldUsername + ' changed to ' + @NewUsername + ' in NTForums_Posts.'