Can I delay my DTV visa application for training in Thailand until September?

Concerning the DTV: I'm in America, and the Muay Thai organization has packaged their letters and sent them to me. The letter states that I want to start training in June. On the Thai Embassy visa application, it asks for the flight information and hotel/accomodations information. They ask that entry should be within 3 months and not longer.

I have not purchased a flight or reserved a hotel yet, so my visa application is in saved mode. My bank has issued to me a written statement for required funds, dated April 11. My question is can I delay my return to Thailand until September? If I delay the application, will my bank and the Muay Thai class need to change their dates? If I book the flight for June and reserve a hotel, then apply and get approved, can I cancel the flight, change the flight date to September, will I still have a valid DTV 4-5 months from now? It is possible for me to leave here in July, or earlier, but I have a financial opportunity that's difficult to put aside. Any suggestions will be appreciated.