As the seat reservations are optional and free of charge you can make a reservation for the train you intend to travel with and then make another reservation if there are changes.

The UK experts will tell you the best way to make the reservations. 

You can reserve trains on that route from www.lner.co.uk that will also show show a seat map so you can see how many seats have been reserved. Just add 0 when asked for a reference number.

LNER normally operate their trains with just one unreservable carriage and so at busy times (Most of the morning trains and any from about 1600 to 2000) there is a reasonable chance you will be standing or seated in one of the reservable carriages and playing musical chairs as people at stations up the line can reserve as the train approaches.

As earlier posts have said reservations through UK operators are free of charge and certainly LNER allow you to change your seat if you don’t like the one allocated. The only time you will have to present a reservation is to eject a squatter, which is your right of course.

ok I made the seat reservation and just added a 0 on LNER website. Scan the pass at the station and then present the reservation onboard if necessary.  I keep an eye on table tables leading up to the occasion to ensure they remain the same

Within the UK the scan doesnt worked I always had to show my pass to the guards on the gates to let me on the platforms :)