Remove mixed extents in SQL Server
Plain answer is yes, if you have enabled trace flag 1118 on your instance a rebuild of those indexes will move them to full extends instead of mixed. You can get more information on this from Paul Randal's post: Are mixed pages removed by an index rebuild?

A synopsis of the comment raised by ray herring on July 10, 2014 and Paul's response:

Ray asked if enabling this flag, and performing rebuilds would move all the pages in mixed extends to dedicated extends. Paul stated yes it would. However one thing I would note is his second question, on if it was worth worrying about for existing tables and indexes. Paul stated, no, since mixed pages only really make a difference in performance is in tempdb. 

So you are not likely going to see a great deal of performance increase by worrying about the current pages that are mixed. Just do your normal index maintenance and at some point they will get moved.