Since nowhere is stated you can lower the amount during the visa validity period, like mentioned at non-o marriage and retirement, the 500K is just the requirement at any time. It only will be checked now at approval and extension time, but it can be checked anytime.
I still find it laughable that some people here claim that an advantage of DTV over non-o is that you don't need to have money in a Thai bank account, which is not a requirement for non-o either, and then complain that with a DTV they can't have a Thai bank account, which they hate so much.
Obviously they want an account, but not want to have money on it, so completely useless for banks.
Mr 5 Bank Accounts Why should banks facilitate the loophole that you can come back immediately after leaving. The maximum stay is less than a year, and when I read here most stay less than half a year.