From what I know, the new rule in BKK is all Fast Track card will be given by the BKK ground staff upon opening of the aircraft door. Number of cards given are usually exact with 1 or 2 spares.
You may try your luck using the Fast Track card with your non premium class ticket as some times, the staff will check and will reject you. Some DMs i heard from was turned away at the premium lane.
The new rule applies to all airlines, not only CX.