in country extension is not requalifying, they only check if you still comply with the requirements. Immigration can ask at any moment they want, ask you to show you comply with all the requirements. Switching between category is not documented, so you should request immigration about that. Strictly you do not comply to the purpose for which you where approved when you change, so it could be considered invalid.
Anonieme deelnemer 722 the 500K is ALWAYS required, also when exiting and go back, it just is not CHECKED, but that does not mean you don't need to have it.
Anonieme deelnemer and less USD 1000 is not enough to support yourself in Thailand, so if you not have other income or financial reserves they might reject the application. But how do you plan to survive on less than 1000 dollar?