I'am not sure if you can do that. the only way you can do it and it not exactly the way you want it is scripting. and the other way involved coding.
scripting involved to set up a scripted scene to change the diplomacy stat but that it not what you want.
coding is to change the in game hard code for the AI behavior but i have no idea where to start.
maybe their is someone who know a easier and a better way than me.
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