Well, Tom, this POD presupposes that as in Cambodia and Laos, a substantial largescale indigenous Communist guerilla movement emerged among the ppl in northern Thailand thruout the course of the 1960s. Thailand was a strong US ally during the VW, sending combat troops into action against the Commies and granting basing rights to the USAF from Ubon and U Tapao, but I'm not too sure myself of the strength and composition of any indigenous Thai Commies during the period, although they were doubtless running around in the jungle as per everywhere else in SE Asia.
1 potential POD I can think of is if the Thai Commies got strong enough somehow to seriously harass govt forces in the north, were answered by brutal but inefficient pacification efforts by the Royal Thai Army which drove the local ppl into the arms of the Commies- perhaps with the addition of USAF bombing of local villages as per Cambodia and VN which alienated peasants so much they went into the arms of the VC- and the Commies by 1970, supported by a constant flow of supplies from their patrons in VN and Red China, are able to establish effective de facto control over the north, while non-Communist govt forces are compelled to operate from the south. How's that sound ?