Can I withdraw my 800,000 THB while on a retirement O visa if I provide proof of monthly income instead?

And as this is a Theoretical question I close the commenting. If you apply for an Extension of Stay you follow up the rules and requirements of that Extension of Stay. As this requirement of seasoning money in the bank started on 1 March 2019, there are no answers what really will happen if you do not follow up this rule. My advice is follow the rules and keep the money in the bank as set in the requirement to be on the safe side

Your logic is wrong. You have an Extension of Stay based on being over 50 years of age and met the financial requirements for this. For your next application they will check if you kept your promise to keep the money in the bank as you where told to do so. If you break that promise you will simple get your next application denied.

You can do what ever you want too BUT remember you signed for the conditions of your extensions. I would expect an issue the next time you want an extension.

Follow the Thai Immigration rules if you wish to continue living in Thailand.

...If you are using the 800,000 Thai baht banked method to obtain your yearly extension of stay, during the two months before your date of extension 800,000 needs to be in your Thai bank account and remain their for first three months. After which the balance is not to go below 400,000 Thb.

...If you do not, you may be refused your next one year extension.

...If you use the 65,000 Thb international deposit method, you must do so all 12 months each year to qualify for your extension.

.. You do not show your bankbook at the airport. You show it, if asked, at your Thai Immigration office.

...Some...a few Thai Immigration offices are requiring the expat to return to the office 90 days after the extension is approved and show updated bank book with at least 400,000 Thb balance.

...OR RETURN to your home country and obtain the O-A non immigrant Multi-entry visa for retirement. No money is required to be in a Thai bank. Just meet the simple visa requirements and with planning the visa gives you up to two years of living in Thailand.

Also, it depends on what Immigration office you are reporting to. CW in Bangkok doesn't ask to see a bankbook at any time until you apply for an extension.

Banked money method. Monthly income method. Your choice when you apply for your 1 year extension. You just have to meet the requirements.

your logic is right -

You poke the tiger prepare to get bit....