@Chris *********
It is mentioned as a visa requirement, I asked immigration and embassy because I wanted to know what exactly is expected and if I can put the money to another account.
I got answer it is a requirement for the duration of the visa and completely up to IO at the border if they check or not, and that I am completely free to have the money on another account.
When I approved there also was no need to show you already had the funds before approval, but there was a remark that embassies can ask for additional documents.
So it are not requirements itself that change, but the wya you have to prove them.
It is not that difficult to understand why the 500K is necessary and it would be highly unlogical you need it only when you are not in Thailand, but normally in your home country, and then as soon as you travel you don't need it anymore for the first months, and then when you want to stay longer you suddenly need it again.
The fact that you normally can enter many times without any checks does not change the fact that it is a requirement.