If you just booked it, you might have to wait for a few days for the IT to recognise it all. If you then can book seats, fine, but if there is an aircraft or schedule change then you might lose the seats. Either way after your Silver membership ends you will no longer have the ability to change it. Though since you would drop to Bronze you can do it from 7 days out.

PS Obviously, if after a few days you can’t do it online you can always call in to do it.