You should to go to your local immigration office and cancel you current visa. Tell them what date you are leaving Thailand and they will cancel the visa for that Date. This is to be safe for when you re-enter with your DTV
I applied for DTV last month, Freelance consultant. HCMC, Vietnam. The consulate did request to see examples of my freelance work, and also evidence of freelance work coming up. They did also ask to see my CV, qualifications and professional portfolio. All of which was not a problem for me, and I successful in my application