I agree with you, but what I mentioned at the end was that ‘Immigration officers are the best people to answer that.’ When someone asks in the group, what we can offer are our own thoughts and experiences but at the end of the day, we’re just like them(non-imm).
I personally don't think that DTV visa holders have the privilege to change to a Non-B Visa. I think the best option for you is to leave the country, apply for your new visa, and re-enter with that visa. The DTV visa will either be automatically cancelled or can be cancelled later. Immigration officers are the best people to answer that—wait a few more days for a clear response.
Basically, I have been doing since more than 3 years via tm47.immigration.go.th. I just applied for mine a few days ago and am now waiting for it to get approved. You just need to fill out the information very accurately and submit it at least 10 days in advance. That would be the most suitable time to do it.
I think you need to get those documents. Just visit the Embassy of Nepal in Vietnam, pay the service fee, and obtain the documents. It's not something I'm wondering about.