It's actually quite simple.
The first thing to remember is to make sure this is the LAST thing you do on the account before transferring. You will need 1 vehicle in the garage that will not be changed into a next gen car/bike, also make sure after performing these steps, you force the game to save before logging off the last time.
You need a mod loader that has Garage Editor and AP ii Intense (AP ii is the best for modding your car)
1 - BUY an ADDER (don't spawn or use FREE options, this way you can sell it later if you decide to)
2 - Upgrade and paint to your liking (don't worry about things you are going to change later, like rims)
Drive the car out of the garage and back in to make changes stick.
3 - Open garage editor, select the garage with the car, then select the car you want to convert to a DONK.
4 - First make sure it's insured, then set "Destroyed" to "ON", do not leave the menu (you will see the car disappear)
5 - Go directly from the "Destroyed" option to "Replace" at the top and select.
6 - Select "Input" at the very bottom of the list and manually type in "faction3" and save
7 - Close the menu and go to the blue dot used for moving cars around and enter the menu.
You will see something that says "Downloaded content not available" or something like that.
8 - Swap that with the car that won't be changed and then swap it back.
9 - Close the menu and do the next car/bike
10 - Transfer the account and call Mors Mutual to get all of your cars back.
TRUST me, this is a lot easier than it looks. Things I've learned is that this should be the very last thing you do and DO NOT go back into this account on the PS3 until after the account has been transferred. If, for some reason, your game locks up before you save, you WILL have to redo everything with converting the cars.