@Kevin ********
You show up here on a visa exempt or tourist visa entry, you make sure where you're staying in Chiang Mai registers you on the TM30 program (notification of foreigner in residence) you go to the Chiang Mai immigraiton office and take a picture of the wall that has the documents you need to apply for an in country 90 day Non-O visa. <- the O means "OTHER reasons; marriage, retirement, raising children, volunteer, trailing spouse, etc) You will be getting it based on being over 50 (retirement)
IF you enter visa exempt get the TM-87 application for visa form, if you enter on a real 60 day tourist visa get the TM86 application for change of visa status.
The form is the same what differs is the requirements you meet to get the visa issued.