0) Start game from scratch (not just load game) and load a save before The Exchange
1) You need game to put a vehicle you want to make AP in slot 1 of vehicle pool.
2*) You may rely on luck, or hope to make a random vehicle AP/FI, but in most cases game will just have slot 1 empty and select slot 2 then (which contains El Train from Portland, which always exists and is always in slot 2). I have made an AHK script (linked below, only works on version 1.1!) that allows viewing if vehicle slot is occupied in memory, and which model is used. Use a replay and then spawn a new vehicle. Check if it is in slot 1. If it isn't, try again.
2**) If you want to use vehicles already in your garage, then, if you have 1 vehicle in garage, press F1 and open garage immediately. Vehicle should be placed in slot 1. If you have 2 vehicles, you'll need a script to make sure vehicle you want was selected (if it wasn't, close, press F1, open garage again). If you have more, you need script for checking + you will have to manipulate vehicles in garage in addition (remove vehicles from it, close, open, add, etc).
3) Let's assume you have a vehicle you need for conversion in slot 1. Now drive to The Exchange mission marker. Leave your vehicle in safe spot so it doesn't despawn. Start mission as you want, but when you are about to leave mansion area (when normally helicopter spawns), activate a Shoreside replay with slot 1 empty (example of such replay is linked below) and continue spamming it, until you are sure you've left the area. When cutscene starts, it will show your vehicle instead of heli. If it shows heli, restart the game. If it doesn't show anything and there is a red blip on radar far away, you had a bad replay, make a new one or use one linked below and try again. If it doesn't show anything and there is no blip, then you were spamming replays too long.
4) Now that you are in a vehicle, drive it to garage, store it. Now go to the streets and look for a vehicle with red corona behind it. Use F1 replays to reset current vehicle pool slot position. When you see it, get into it and don't let it despawn by any means.
5) Wait for timer to run out. When it does, the vehicle with red corona disappears. You can now save and reload the game. The garage will contain AP (FI) vehicle.
Link to said AHK script: https://cdn.discordapp.com/attachments/373791854224867330/414755938554347521/pool.zip (reminder, only 1.1 version of the game! You need AutoHotkey. It crashes the game on game reload, so close it first).
Good replay for conversion: https://cdn.discordapp.com/attachments/150665608982298625/414800381797859340/replay.rep
Credits: blacklev for original idea, Powdinet for original video and confirmation this works: https://www.youtube.com/watch?v=6PHFSWHov1U and Patrick for original AHK script