@Brook *******
You will need to keep the 800K in the bank account you used to get the retirement extension.
THERE IS NO WAY AROUND IT
You are NOT allowed to move it, if you do, Immigration will revoke your Extension of Stay Permit and you will be in overstay beginning from this day
After being issued the Extension, the 800K must remain in this account for 3 more months, and shall never go under this amount
For the rest of the year, a minimum of 400,000 THB need to remain sitting in the account
Then again, two months before the next application, 800K need to be seasoned for 2 months on the day of application
You are NOT allowed to move it to another account
You COULD ask your Immigration if they accept that you move the 800K into a fixed deposit account, but I HIGHLY doubt they will accept it