I really don't want to seem dense, but being a data analyst/DBA I just checked the users table and it appears user_id is not set to autoincrement (which is the only way that this mod would work). I still fail to see why it would be useful to change a user's id to whatever you want, but at least there won't be any ID collisions.
However, what about updating tables that are the results of other mods that have a user_id column? Is there the equivalent in mySQL to information_schema.columns where you can look for a column like '%user_id%' and update that?
Thanks for your comments.
In fact this MOD is not used for auto-increment. It simply changes the userid of any existing member to another one if and only if there is no collision.
This MOD updates the relevant tables' user_id columns.