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.