Hi,
This is normal for Tg FA´s. The second uniform is a landing uniform. Just imagine the staff taking a taxi or a car ex airport back home with their elegant Thai dresses! Or indeed walking the distance between the airplane and the taxi stand or car park!!! The male attendants also have a change of uniform. From memory I think its mainly a jacket change but I could be wrong. My attention to the details of the female attendants is however more interesting.
Cheers!