If you fail to have 800k in your bank account at the time you apply for the second extension you will be denied as you did not satisfy the conditions of the 1st extension even though you are swapping to the monthly payments.
As John correctly stated the main applicant needs 2 years of passive income of $80k to qualify for the LTR-P visa, if you are applying as a dependent and not for your own LTR-P visa you can apply based on the main applicants visa.
Just to clarify the LTR-P financial requirements, you each need to demonstrate $80,000 USD passive income for the last two years before applying for the visa.
“Applicant must have a bank deposit of not less than 3 million Baht in Thailand or a bank deposit of not less than 1.8 million Baht and an annual income of not less than 1.2 million Baht per year. Such amount must be maintained in bank deposit in full for at least one year and kept at not less than 1.5 million Baht thereafter.”
US Thai Embassy
I guess it depends on which embassy website you read the requirements.