On 8/23/2019 at 8:46 PM, NightSky said:

Is it really OK to simply carry both passports through border inspection at Suvarnabhumi and then transfer the extension stamp at immigration after im back in BKK?

Yes it’s okay. Visas etc. stay valid even if in cancelled passport. Just show both passports and make sure the IO sees your extension and re-entry permit.

 

Once in the country you go to immigration to get the stamps transferred to your new passport.

On 8/24/2019 at 2:46 AM, NightSky said:

If I obtain a new passport whilst in my home country the passport office will cut the corners off my old one making it obsolete and officially I will no longer be able to travel on this passport. However my extension of stay and re-entry permit will be in the old cancelled passport.

 

Is it really OK to simply carry both passports through border inspection at Suvarnabhumi and then transfer the extension stamp at immigration after im back in BKK?

Your extension will remain valid in the cancelled passport. You will show both passport on entry to the country to use your re-entry permit for entry to the country and immigration will stamp you into the country in your new passport. 

Then you need to go to immigration to have your stamps transferred to the new passport.