Do I need to worry about this already being fully booked as the trains will probably be or should I be ok to turn up on the day and get a seat?

I am flying home to Australia from KL in Malaysia on the 6th of Jan. and am hoping to get a flight from Bangkok around the 4th to KL, however I most likely will be celebrating new years with friends a considerable distance away from Bangkok meaning I'll probably be trying to hop on a bus on the 1st, 2nd or 3rd - there isn't much margin for error!

I'm not sure exactly where we'll be for new years (maybe Chang Mai, I'm not sure) so itd be good to get a broad idea. I'm not really fussed about government bus or tourist bus and will pay a premium price if I really have to.

Thanks!