You have to call the MilagePlus premier priority desk to make this change. As premier gold you don’t have to pay any change fee to change the departure time. But, if you layover in BKK is more than 24 hours, I don’t think they will allow the change. I don’t think you can have a stopover for a one-way award. You should ask this question in the UA forum.

I have done a slightly different change for my award trips many times. Every year I book SFO –SIN, 20+ hour layover, SIN-CMB and then return CMB-SIN, 2 hour layover, SIN-SFO. But then I have changed my return CMB-SIN leg to one week earlier so that I have a week in SIN. Then I do a side trip from SIN to somewhere else in the region. But for a return award trip, you can have one stopover, which you don’t get on a one way award.