The question is more like how much to set up a company and requirements to get a Non B. You’d probably need to get an accountant or lawyer to do this. Estimate cost about 60k but that could be wildly off the mark.
If you use an agent to bypass the funds requirement then you’ll be stuck on the agent train until you enter on a clean stamp and start afresh.
If you use an agent just to push paper and bypass queues but fund the visa/extension yourself then there would be no reason that you’d have to use them in future.
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.