the Royal Gazette entry that detailed the DTV specifically stated it could be changed to a different visa type inside Thailand just like the other tourist entry stamps
the money needs to be in a Thai bank so it's baht that matters, not dollars. It will sit there a lot longer than that, the rules say it has to be there 2 months before, 3 months after and can't go below 400k at any point in the year. Your money doesn't do nothing but interest rates in Thailand aren't great.
it's to show you have the money required for the extension of stay in your thai bank account for the full two calendar months needed in order to apply for the extension of stay