rewrite custom field on user profile I have a custom field on my user profile pages where users can enter their Twitter user name. When the user's profile is rendered (either on the user profile page itself or via views) I would like to rewrite this field so that it links to the user's twitter profile following the pattern http://twitter.com/[field_twitter_name]. 

I discovered the twitter username project on DO yesterday (https://drupal.org/project/twitter_username) and this would help me accomplish the objective, especially for new users. However, we currently have ~900 active users on our site. 

Is there a way to create a new field and update its tables with the existing user's twitter names? Or is there a way to change the field type from it's current setting of "Text field" to the new type made available by the twitter username project "Twitter username"?

Any help would be greatly appreciated.
You could write a small one-off DB copy function to accomplish the task. There a couple of modules around but they appear to be quite old, so it would depend on what version you run. Here's a tutorial for MySql (if that's what you use) DB access: http://www.w3schools.com/php/php_mysql_intro.asp