It may be possible if when you go in the trigger zone instead of changing the enemies reactions of you it deletes the old enemies and replaces them with hostile ones.
An easy way to do this would be to make the friendlies have no weapon ( i.e. Colonel X (Unarmed)) and name him "001" then have a basic script to have him dissappear. Then, have enemies that have weapons but still the same type of enemy (i.e. Colonel (Pistol)) also name him 001, and set their stats to the following:
Spawn At Start - NO
Max At Any Time - 1
Maximum Spawn - 1
Number To Spawn - 1
Just place those over the unarmed ones.
Then, put a trigger zone where nessacery and set it's "Main" colloum to the script "plrinzoneactivateifused" then put in the "If Used" colloum to the enemies name (in this case 001) and that's it.
About the "basic disappear script" just search the forum for that (search like "trigger zone" or "switch") and place it in:
C:\Program Files\The Game Creators\FPS Creator\Files\scriptbank
It should work but tell me how it is.