Once you try to go airside again and pass Thai immigration the officer is supposed to check that your BP is issued at BKK. If not, you will be send back to check-in to have a new BP and will most likely have to pay Thai dep. tax, THB 700. However, it has been reported many times in this forum that imm. officers may not notice and allow you to pass (as if you had a BP issued at BKK). I guess it is what you could call a "hit or miss". Good luck!