I hold retirement extension valid till December 2025 and multi re entry on my UK Passport. Going to UK in March 2025 and will get new passport on return. I will carry both passports when I arrive in Bangkok and presume Immigration officer will stamp new passport with extension based in old passport  till December 2025 ? Please correct me  if not ? What is the procedure after that at Jomtien Immigration to transfer all stamps and how/when to do it ?  All advice will be appriciated and I thank in advance.

Yes return with both passports.

What you outlined is correct. 

BTW: where did you obtain the reentry permit. Jomitien? 

You can transfer the stamps anytime.

Are you using money in bank method for retirement. 

https://aseannow.com/topic/1245494-entering-thailand-with-new-passport-and-visa-in-old-passport/ 

 

https://aseannow.com/topic/1171201-transfer-of-stamp-to-new-passport-at-jomtien/