What is the procedure to convert a Non-O (Retirement) 1 Year Extension of Stay into a Non-B (Teaching) 1 Year Extension of Stay in Thailand?

That's why it is easier to stay on a Non-O marriage extension rather than retirement, with that you can work, with retirement not.

I don't think you're going to be able to go straight from a yearly extension of stay based on being over 50 (retirement) to a yearly extension based on employment :/

I think it is going to be a multi-step process that goes something like this

1- you cancel your current extension of stay based on retirement at your immigration office

2 - You immediately apply for a 60 day "unable to travel due to covid" extension for 1900baht & you get an under consideration stamp.

3- when that date comes up you go back to the immigration office and get the real extension inked into your passport

4 - then you apply for an in country 90 day Non-B visa based on employment with documentation from your employer, pay the 2000baht and get an under consideration stamp

5 - when that date comes up you go back to the immigration office and get the Non-B visa and new 90 day stamp inked in.

6 - when that 90 day stamp runs down to about a month left you go apply for a yearly extension with documentation from your employer for 1900baht, you'll get a 30 day under consideration stamp

7 - when that date comes up you go back to the immigration office and get the year extension inked into your passport

That is how I believe you'd have to do it to pull it off inside the country at this time..