Can I apply for a retirement visa in Thailand using a different passport without leaving the country?

The process works exactly as I outlined it AND remember just because you stamp OUT of thailand on an extension or even if it runs out while you're out of the country that doesn't officially cancel it

If you are on an extension based on employment you need to go to the immigration office that issued that extension and cancel it BEFORE you leave the country.

This is the process

1 - Cancel the current extension of stay

2 - leave the country on that passport

3 - re-enter the country (by AIR) on the new passport free stamp entry

4 - get the 800K baht in a thai bank account in your name only

5 - apply first for the 90 day Non-O visa for 2000baht at the immigration office where you live

***wait the require under review period

6 - go back get the Non-O visa and new 90 day stamp inked in

***wait until the 800K baht is seasoned 2 months and you have 30 days or less left on that 90 day stamp

7 - go to the immigration office and apply for a year extension for 1900baht

As a rule most offices issue extensions based on retirement the same day you apply for them, so once you get the year extension inked IF you are going to travel make sure you buy a re-entry permit

I deleted a comment of yours and closed the commenting until you messaged me because we have a zero tolerance for mentioning agents/agencies/lawyers OR pricing that is not what immigrations charges 🙁

Nope you can't do it inside thailand

You would cancel the current extension you're on in the one passport,

You'd FLY out/back in (as you can only swap passports by air)

enter the country on the new passport and starting the process of getting the initial 90 day Non-O at the immigration office using that passport