I already train at a Muaythai gym in Pattaya, but I don't think they are set up to help with DTV.
1. If I have to change gym, what's the minimum period of enrolment for a successful application?
2. Do I have to re-enrol at the same gym? Would I be able to re-enrol at my USUAL gym even if it's not the one on my original application?
TLDR : Answer Summary
To successfully apply for a DTV visa in Thailand, you must enroll in a Muay Thai gym for a minimum period of 6 months, based on the comments provided. You do not have to re-enroll at the same gym as originally stated in your application; instead, you can join your usual gym as long as it meets the DTV requirements.
DTV VISA RESOURCES / SERVICES