Can I switch from a volunteer extension to a retirement extension in Thailand?

you're most likely not on ANY visa especially if you were getting them at the immigration office every year 😮 if you look in your passport you should see the current stamp says "extension of stay permitted until" and that's what you're actually here on, and extension which is NOT a visa.

All you will be doing is switching the reason for your extension to one from volunteering thru that legit foundation/organization to a new yearly extension based on being over 50 (retirement) you'd need to have the 800K baht in a thai bank account for 2 months before you apply for the extension (not before your current extension runs out but 2 months before the date you will APPLY for this new year extension based on retirement)

You will probably need a letter from the foundation stating that on xxx date (usually the date your current extension runs out) you stop volunteering. You can go with that letter, they'll cancel the volunteer extension and you immediately apply for a new extension based on retirement by meeting the financial requirements, documents etc

GO to your immigration office and show them the extension you have now tell them you want to CHANGE THE REASON, (do not say you want a retirement visa because you don't and that will screw them up and they'll tell you to leave) all you're doing is changing the reason for an extension from volunteering to retirement

It seems you are not on any visa at all.

I guess you visit every year an Immigration Office with documents of the organization and get every time a new stamp with the text " Extension of Stay permitted up to xx/xx/xx" .

You go with a letter of the volunteer organzation to the Immigration Office to cancel your stay at your last day of being a volunteer and apply directly for a new Extension of Stay based on being over 50 years of age (retirement).

Make sure you meet all the requirements for the new extension before you cancel you legal stay in Thailand.