I have a question for the panel. The last time I entered Thailand, a number of years ago, I entered on a single entry Non "B" visa. Since then  I have been applying for and receiving a 1 year extension of stay at my local immigration office. Retirement is not too far off - 2-3  years - and I am considering changing the reason in my application for my next extension of stay from employment to guardian of my children as this will be the reason used after my retirement.

Can this be done without leaving the country simply by changing the reason on the forms and meeting the requirements or would it be necessary to leave Thailand and return with a Non "O" and then apply for the extension?

Yes you can change the reason for you extension to another one.

Not sure what you meant when you wrote guardian extensions since those technically do not exist.

You can get an extension based up being the parent of a Thai or the parent of a foreign child attending school here.