Just received my new passport (old full) but don't really have time to go to Chaeng Wattana to transfer stamps before flying out on Thursday. Is it really requirred or can I show my old invalid passport and can get exit and entry stamp in new? Why am I requirred to show up in person at immi when transferring stamps, yet ni need for 90 day report or picking up extension of stay?

If you do it at the airport they will only stamp your permit to stay in new passport and departure stamp. When you return you will need to show both passports to use your re-entry permit in old passport.

On return you will need to go to immigration and get the remainder of your stamps transferred.