so when you are on a tourist visa and apply for your O it cancels the tourist visa? I am glad you are sure, lots of people are sure and wrong at the same time.
20kthb is if you come visa exempt. You are supposed to have cash with you so immigration can see you can support yourself. It is one of the things they can ask to see.
For the DTV you are showing you have access to that money and not going to be trying to find a job in Thailand.
If you extend you will probably be asked to show that you still have it. You might be asked to show proof anytime you go through immigration.
Probably not at the airport, but they can if bored or just don't like you. Just like the 20k cash they can ask you to show.
They are cracking down more and more and the DTV is under more scrutiny. Pretty sure being caught no longer qualifying is going to lead to a bad day and possible ban. Is the risk worth it to you?