This requires a save in Empire Mode with edited characters and experience enabled.

1. Train your edited characters to whatever extent your wish in Empire Mode.

2. Delete the character(s) you want to be replaced and make your new character(s). Save your change when the game asking.

3. Clear Empire Mode with the save you trained your characters in and save.

All your edited characters will instantly reach whatever rank and stats the characters in your Empire Mode game have, even if the new characters are completely different.