@เเอนเดรีย ********
(andrea) did you get them the year extensions based on being thai OR are you just letting them overstay their entry stamp or what?
I ask because IF they haven't overstayed they can easily get a year extension on the US passport by giving minimal documentation at the immigration office for 1900baht
However IF they did run on overstay (as in stayed past the date they were stamped in when they arrived) without you getting them extensions they can't.
AND
As people pointed out the only way to change nationality is to fly out and back in, it can't be done at a land crossing at all.