How can I transition from the 800K baht option to the 65K monthly transfer for my O-A visa renewal in Thailand?

800 k only

actually you as a canadian CANNOT use the 65K baht a month transfers for the previous 12 months,

That's because your embassy issues the income affidavit notary letter.

You either bank the 800K baht in the account for 2 months before you apply for your yearly extension OR you get that letter from the Canadian Embassy stating you get over 65K baht a month in income from overseas.

Those are the only 2 ways you can meet the proof of funds

Your options are having bt800,000 in a Thai bank account in your name only at least 2 months before, and three months after your first visa extension, then keeping at least bt400,000 in the account for the full year. The second option is to have at least bt65,000 coming into your Thai bank account from an overseas source for at least the last 12 months before you apply. It is not an average. You can not receive bt45,000 one month then bt85,000 the next. It must be at least bt65,000 every month, and on your bank statements it must show as coming from an overseas source. ATM withdrawals, or deposits from inside Thailand are not accepted. Because the Thai immigration started requiring embassies to certify as accurate the notarized letters they had been issuing stating monthly income, many quit issuing these accepted letters, as they were not certifying the amounts listed by the signee were accurate. If your embassy still issues these letters they are accepted, but is is not a requirement no matter what country you are from. The bank statements showing at least bt65,000 came into your account from an overseas source each and every month is accepted. If you use the bt800,000 option the first year, and decide to switch to the monthly option the next year, for that first year the bt400,000 must still stay in your account that whole first year, until you apply again. If you use the bt800,000 option for your next year extension, then your account has to be brought back up to over bt800,000, at least 3 months before you apply, and kept in your account for just 2 months, but you still must keep bt400,000 in your account the full year.

As a Canadian and you you meet the requirements in regards to annual income you can use the Income Statement letter . Go to Canada Revenue Webpage and click on Proof of Income Letter. It is basically a copy of your Tax Returns for the year . Print off a copy and take that to Canadian Embassy or Consulate in Chiang Mai . They review it and if it meets requirements will issue you a letter to take to Immigration. They charge 1200. Baht for the service . Just be aware presently you can’t just show up at the Embassy you have to go on their website and book an appointment.