@Ira ************
as James said, you EXIT thailand on the passport of the country you ENTERED on. So if you're going to bounce him out to get off of overstay and on to the new passport just take BOTH passports and give them to passport control.
Also note that you can't switch nationalities by land (only by air) So you can't go to the border with the russian passports stamp out, and then switch to the romanian one to stamp back in.