The stated benefit of Silver is "free seat selection at time of booking".

If the seat selection could disappear once the booking is made, then this should be clearly stated in the terms. Perhaps I have missed it, but there's no term that says you need to be Silver at time of flight - the key moment is "at time of booking".

I have no real world experience of this situation, but this is how I read the wording.

Perhaps BAEC may be able to assist with a quick call to clarify...?