Is it possible to convert my long standing marriage extension of 24 years to the simpler retirement extension by just showing up for the usual extension and presenting financial reuirements?  I am 76

All you have to do is change the reason for your extension from marriage to a Thai one based upon retirement.

You will need 800k baht in a Thai bank for 2 months or proof of 65k baht income.

Best to do the change of the reason for your extension close to when your current one ends since the new one will start from the day you apply. Immigration will cancel your current extension and then issue the new one is the reason for it starting on that day.

Many thanks Ubonjoe

Do you also have to submit a medical test for the unusual diseases (costly) and the undergo police check (costly in terms of time) in your home country before applying for this? I'm mulling forgoing the retirement application here in the UK and going ahead with the spouse visa and obtaining an extension based on retirement as per the advice above.

10 hours ago, KathuPete said:

Do you also have to submit a medical test for the unusual diseases (costly) and the undergo police check (costly in terms of time) in your home country before applying for this? I'm mulling forgoing the retirement application here in the UK and going ahead with the spouse visa and obtaining an extension based on retirement as per the advice above.

The medical certificate and criminal background check is only required to apply for a Non-OA visa at embassy or official consulate.

Applying for a Non-O visa based upon marriage to a Thai does not require either one of them.