I was on an O with a multiple re-entry permit. To get a B I had to leave the country and get the B at an Embassy. As other have said. Ask immigration if they will convert in country.
People reported doing that run just last week. There have been adverts in Chiang Mai groups offering it for 2500 for DTV holders. No need for overnights etc
Vol Visas were so abused they do not issue them any more. Even credible NGOs and charity organisations can not get them even with backing of Thai politicians etc.
I feel it is a double edged sword with CM Immigration. They can be sticklers for paperwork but on the other hand they are the most helpful immigration I have dealt with in Thailand - and I have dealt with Jomtien, Chimchurri One Stop Shop and Bangkok main immigration.