No.

I understand that seat selection works through a remark in the booking containing the FFP number and a link to a central database verifying that the FFP number is eligible for free of charge advance seat selection.

This remark will be completely unrelated to either the U class reservation or the ticket, because the ticket is an entirely separate document that is also simply referred to on the booking but does not of itself form an integral part of the booking.

The BAEC account is only related to the ticket, because this is where the miles that were used to pay for the ticket originate. If the BAEC account did matter for the purposes of advance seat selection it would be a bit like saying that if you buy your ticket in GBP you can select a seat but if you then cancel the ticket (but not the booking) and buy another ticket for the same booking in EUR then you would not be able to select a seat.

So it should be easy to change the FFP number.

Does that make sense?