#user

I was able to bypass the issue (that looks like an incomplete installation of an old version of private_files_download_permission) by dropping the foreign key constraint before running the upgrade, then recreating the foreign key after updating the private_files_download_permission uid to match the users uid.

I tried the custom module but got:

SQLSTATE[HY000]: General error: 1833 Cannot change column 'uid': used in a foreign key constraint 'private_files_download_permission_directory_user' of table 'TABLENAME.private_files_download_permission_di   
  rectory_user': ALTER TABLE "users" CHANGE "uid" "uid" INT NOT NULL auto_increment; Array                                                                                                                     
  (                                                                                                                                                                                                            
  )