What should I do about my Thai visa if I'm renewing my passport and it was issued on my old passport?

Carry the old passport with you and show both, plus the printout of the DTV visa to the airline check in and Thai immigration. They should stamp you in on your new passport. They may also make a note that your visa was issued on your previous passport (noting the passport number) if they don’t then you can get that done at an immigration office. Thereafter, you can travel with your new passport plus a copy of your old passport along with a copy of your DTV.

I paid 500 baht and got the stamp transferred into my new British passport at the immigration just outside Hua Hin for Prachuap Khiri Khan, after speaking with an immigration officer at the smaller office in Bluport Mall.

Keep both passports. I have the same issue, 10 yrs US visa is on my old passport.

Carry both passports

Take the old passport with you