Board Thread:Support Requests - Designing Your Wiki/@comment-4848837-20130212204540/@comment-3448772-20130212220251

Yes there is a way to do so. There is a media wiki page that you can edit to change it, however it will only change the user pages of new users, but it will not change unedited ones.

You can edit it at (MediaWiki:Welcome-user-page) on your wiki. For future notice you can change similar wiki messages by finding them at (Special:AllMessages) on your wiki.