So does this guy need to apply for the log book to see if it arrives and if there are any issues for the Y plater.

If all clear, swap all the bits over and just MOT and tax the car as normal keeping the Y registration/ID

Declare the A plater as scrapped and put the private plate on retention, or have it put onto the Y plate?

What do people do if they replace floor pans in an old car and so in turn loos the id numbers stamped in the floor??