When will the aged 3 car DMUs of SRT be replaced?

I have read that the State Railways have bought a number of new carriages and locomotives but many of the coaches seem to be for use on overnight sleeper trains, but its a good move to try and bring the fleets up date.

The fleet of 3 car DMUs which often seem to called "Special Express" also need to be replaced, they are almost 30 years old and look and feel like they are life expired, does anyone know if or when this likely to happen? When I look on the SRT website they still show the old DMU for service on the 0850 Chaing Mai to Bangkok although the other trains on the route bring up no sample pictures.