yes it is correct. You have to fill the previous extension requirements in order to get the new extension. Otherwise people would not keep the funds in the account as required.
You should bring the account back up to 800k 2 months before the next extension EVEN if you are switching to income method.
By doing this you guarantee that you will get an extension IF for some reason they deny the income method.
Any sort of mistake, or what can be interpreted as a mistake could cause you to start all over again.
For the sake of a couple more months this is the safest practice.
So the answer is are you cautious or are you certain you fully are qualifying for the income method?
I've seen extensions denied because of the money not transferred close to the same date every month and some that dropped just a few baht under because of exchange or fees.