It's not so much that the Thais backed the wrong horse as that the Japanese have been strong-arming them.
Well, it is not yet clear, at this point, whether the Japanese is still in a position to still strong-arming and force the Thai army to became part of the Japanese defense.
Meanwhile, the fact that they did not yet secure the position in northern Malaya (specifically,
the territories that Thailand (
re)annexed IOTL), would probably meant that
Phibun would not have gained the public support that IOTL was large enough that he was acquitted by the end of the war.
As for the Thai military, they could have some casualties as a result from the upcoming battle. But more importantly, the fact that the British is in a much stronger position in regards to Thailand, could meant the size of the Thai army could be reduced (as was being demanded by the British army IOTL IIRC), and stay relatively (or perhaps, significantly) smaller than IOTL thanks due to the likelihood of an communist insurgency in the neighbouring countries (and thus US aid and support) being reduced significantly. (Thailand losing
Sadao Sendawa and
Tak Bai Tabal to Kedah and Kelantan respectively, let alone losing
the whole Patani region is still definitely in the realm of ASB though)