This was a fun little exercise in Google Apps Script to essentially transfer ownership of Google Drive files from one user to another.

Here’s a script that the old user should run to change ownership of those files to the new user:
ChangeFileFolderOwnership.js

And here’s a script that the new user user should run to remove the old user as editor from the newly changed-ownership files:
RemoveEditorFromDriveFiles.js

Note: You cannot use the file.removeEditor() command to remove yourself as an editor, so that actually has to be run from the new owner’s account.

Another Note: Just as when you change ownership or add editorship manually, scripting will also produce a ton of emails to the new editor/owner that you’ve add that person, so the new editor/owner may want to set up some temporary Gmail filters to mark those as read and put into the trash.