How to move files already uploaded to a new subfolder? I've got a bunch of files in sites/default/files that I'd like to move to a sites/default/files/subfolder to avoid a big mess.

The fields uploading files have been configured to put them in the right folders in the future. What I'm looking for is a solution to organising files already uploaded.

Any suggestions?
It sounds like you're looking for File Maintenance. 


  Ever wanted to move files from your sites/default/files directory (or
  whatever you have configured) to a (new) subdirectory? Never did it
  because you knew it's a damn mess since you need do move the physical
  file, adjust the database files table and look through all the
  node_revisions table and other content to adjust the link to the file
  so as not to break your site?


That should cover any files that are managed under the files table (pretty much any field you should have). If you have any other files under sites/default/files that aren't managed by Drupal, then custom redirect htaccess rules should cover those files.