Though totally ignored by the English press, in Thai it had quite a lot of reports on the proceeding of the legislation. On Thai blog posting which summarizes the open issues quite well - at least as far as I can understand it with the help of Google Translate.
First, the draft law said that Nong Khai constituency 2 will become the constituency of the new province Bueng Kan, which would mean that no by-election in the two constituencies is necessary. However, as one can find in the 2007 constituency definition (Page 27, Item 53), this constituency includes the district Fao Rai, which will not be included into Bueng Kan province. Therefore, to be exact the 50,000 citizen of Fao Rai may require an by-election - even though the parliament might get dissolved early next year anyway.
The other issue is with the senate, which consists of 150 senators, 76 of them elected in the provinces and the remainder being appointed. Now with the increase of number of provinces, will the number of senators be increased to 151, or will one of the appointed senators be forced to resign? The official translation of the constitution however makes it clear that there is no problem right now:
Section 111: The Senate shall consist of one hundred and fifty members to be elected from each province, one member being elected from each province, and to be selected in the number equivalent to the total number hitherto stated deducted by the number of elected senators. In the case of an increase or decrease of the number of provinces during the term of office of elected senators, the Senate shall consist of the remaining senators.Thus, the senator elected from Nong Khai will remain in office until the end of term of the senate, and only then the there will be 77 elected senators. It will only become tricky if the Nong Khai senator Khajon Saiyawat (ขจร สัยวัตร) resigns or dies, as this case seems to be omitted from the constitution. Only sensible answer would be to hold a by-election in both Nong Khai and Bueng Kan together, as if the split of province hasn't taken place.