I have not fully tested this technique and you can loose your HAs which will require a reinstall of
WOFF the latest patch if you make a mistake (not the entire game, thankfully).... you have been warned.
I have found a way to remove HAs from your current campaign then bring them back for another or even same campaign.
Step-by-Step
BACKUP ANY FILES THAT YOU WILL CHANGE OR YOU WILL LOOSE YOUR HAs (
DO NOT MISS THIS STEP) - I was stupid enough to skip this step BTW.
1. Go to the location where the aces text file is stored. I will use Germany for this example, but it applies to any nation.
2. Open the German aces text file with notepad.
3. Find the Ace in the file
4. Go to his Date of Death
5. Change it to an earlier date
Save and close the file.
To reverse the changes: I couldn't simply just change the date back and save the file again. I had to delete the changed file and
replace with my backup copy for him to appear again.
EDIT: Another option is to make a copy of the changed file and rename it to something like "HA file for 25 RFC NOV 1917.txt" . So, when you want to replay that campaign again, you change that file's name back to the aces text file.