Can I change my guardian visa to a retirement visa at Samut Prakan Immigration?

Doubtful you have any visa at all and I would bet if you looked at your stamp you'd see it says "extension of stay permitted until" and has a date. <- That's not a visa, it's an extension which is different

You should be able to change the reason for your extension at the immigration office where you live (Samut Prakan). You'd have to meet the financial requirements for the extension you'd be applying for

Most people wait until their current extension is going to run out and then apply for a new yearly extension for a different reason.

You should go to the immigration office in Samut Prakan, see if they will allow you to change the reason for the extension from raising foreign child enrolled in school on ED visa to one based on being over 50 (retirement). If you can do the change of reason, get the hand out from them listing the requirements, TM30, lease, copies, bank letter, bank statement, bank book etc so you can change the reason

Samut Prakan allows people to apply for a new extension when you have 45 days or less left on your current extension.

Best of luck with it,