In theory you should be able to change the reason, but probably best to check with your office for their rules on procedures.
You would almost certainly need to cancel the volunteer extension with paperwork from the foundation and then immediately apply for an extension based on being over 50.
Ok. You can’t change a Non O visa to a different type of Non O visa, but you can change an extension of stay based on one reason to another.
That said I’m not sure about an extension based on a volunteer Non O. It’s probably going to be office dependent as to whether it could happen in country.
The use of terminology in this case is crucial. Do you have a Non O visa? Or do you have a stamp in your passport that says “extension of stay” - that was originally based off a Non O visa? Correct answers will depend on that.
There’s way more details on what tax you might be liable for. This is why I don’t advise on tax issues. The OP needs an accountant or tax specialist to advise.