Keymap Editor
http://www.tmk-kbd.com/tmk_keyboard/editor/
Use 'Raw URL' or 'URL Shortener' button to get URL of keymap editor and bookmark it. Short URL would be useful to share with others.
Just download and put aside your firmware HEX file for later use.
You can edit keymap of your own firmware file on Editor. You can supply firmware file by URL or uploading HEX file from local storage.
Click 'Load Keymap from firmware' button to load keymap on Editor from your firmware. You can start editting from your stored keymap instead of default.
https://www.youtube.com/watch?v=TYmAb8zOPWU&feature=youtu.be&t=10m59s
https://www.youtube.com/watch?v=TYmAb8zOPWU&feature=youtu.be&t=14m0s