Can I obtain a Non-O visa inside Thailand for a retirement extension, and what documents do I need?

You need at least 15 days left on your current admitted until stamp to do it at most immigration offices (Chiang Mai wants 21 days) because the application goes under consideration for 14 days before you go back and get the Non-O visa and the new 90 day stamp inked into your passport.

If you are using banked money you need to prove the 800K baht was transferred into a thai bank account from overseas. There is no seasoning requirement (meaning you can transfer the money in and then apply for the Non-O visa).

OR you use the affidavit of income from abroad notary from your consulate stating you get 65K baht a month in income.