What it does: Might and Magic 9 contains a list of quest "keys" that indicate what has been done so far. In particular, every promotion quest has the following:
One or more keys to indicate that you have begun the quest One key for each character that is on the quest One key to indicate that you have completed the quest One key to indicate that you have turned in the quest (Initiate->Scholar will have another to indicate what color you chose)
This program will allow you to add and remove the "character X is on this quest" keys by having you select a quest and indicating who is and is not on it. When you add a character to a quest, he/she will be removed from any other promotion quest automatically. If you have not yet completed that quest, you should be in exactly the same situation as if you had added that character from the beginning.
If you have already turned in a quest and need to add a person, the editor will ALSO remove the "turned in quest" key. Simply turn in the quest again to promote the new character. Since this will give additional XP and gold, the editor can be used to remove this after the fact so be sure to write down the values.