My advice would be very much to focus on the now, not what things may be like in a few months. This will help you deal with the changes, which will happen gradually.
When I say it could be the start of the 2-1 I don't mean you need to cut one nap short or anything like that. You only need to try moving the nap a little later first. You will probably also need to move the second nap later too and BT may get a little later too, but then you can start capping one of the naps when you're both ready.
He may well be very tired at the start of nap time, but bear in mind that you want the A time to stay the same or lengthen a little, but you also want morning WU to shift later. By moving the nap later gradually he may start to wake later again.
HTH x