My understanding is FT-ECU is licensed for each ECU. So if your buddy is selling you the FT-ECU license and ECU to which it's licensed then you shouldn't have a problem. I have done this.
If he's just selling you just the FT-ECU license then you are at FT-ECU's mercy on whether or not they will allow a transfer. In your case, going from a 5th gen ECU and license to a 4th Gen I think you are at their transfer rules mercy. My experience with FT-ECU is relying on their goodwill is not a good place to be.
If he's just selling you just the FT-ECU license then you are at FT-ECU's mercy on whether or not they will allow a transfer. In your case, going from a 5th gen ECU and license to a 4th Gen I think you are at their transfer rules mercy. My experience with FT-ECU is relying on their goodwill is not a good place to be.