Today, the 29th announcement of the National Council for Peace and Order of this year (คำสั่งหัวหน้าคณะรักษาความสงบแห่งชาติ ที่ ๒๙/๒๕๕๙) was announced in the Royal Gazette, which by its publication became active law. This announcement deals with the local government administrators and councilors in case of a change in the municipal status. So far, whenever a municipality or a subdistrict administrative organization changes its administrative status, the mayor and the council have to vacate their office and stand for a new election. Especially for the council this is logical, as with the change of status the size of the council changes, thus the previously elected council doesn't fit anymore. Thus, so far usually all the municipal upgrades were done whenever the terms of either the council and/or the mayor ends, and a new election was pending anyway. However, in some cases the time till the next election was quite long, as the Election Commission first has to come up with a new constituency outline matching the new size of the council.
However, since the coup in 2014 there were no elections for the local governments anymore, by announcements 85 and 86 all of the local elections were halted and those mayors and councilors in office were given an indefinite extension of their term by announcement 1/2015. As even for a nation election the schedule is still unsure and depends on the constitutional referendum, those less important local elections certainly have to wait even longer.
I am not sure if this announcement somehow relates with an announcement by the Department of Local Administration from last week titled "แนวทางการแก้ไขปัญหาและผลกระทบจากการปรับโครงสร้างเทศบาล" (way to solve municipal restructure) - which if I understand it correctly suggests that TAO with more than 20 million Baht income should be upgraded to municipalities.