Here's an idea
Realistically you would have to go back to the Anglo Burmese wars and enable the Thais to annex some of the provinces as a reward for their participation with the British invasion (Pre 1900).
Another potential POD although ASB is for the Thais during the retreat of the Japanese Army in WW II. Realising that the writing is on the wall for their erstwhile allies the Thai army sponsor an uprising with a stated goal to rejoin Siam, there is a reason for the Army to annex thethat adjacent provinces in Burma such as the Mon, Kayin and Tanintharyi. How they could hold onto those provinces and then expand is beyond me my level of expertise.
Perhaps a greater influence with Aung San and his peers leading to a Thai leaning regime, hard because of the cultural antagonisms between the two.