ARGH!! I went through something almost identical when I was pregnant with DD2. I had one of the (apparently) most open minded Obs at the public hospital deny my already approved home birth over concerns regarding my BMI. He made me feel like crap. I felt disgusting when he sat there in a meeting with my husband and said I was too fat to go in the ambulance and that they would have to bring one up from Sydney especially for me. I was mortified as this was the same Ob that had read my notes, read my BMI and had written that I was fine to birth at home through the home birth scheme. I was told that if I birthed at home I would probably kill my baby or myself due to my size. So, they broke me and I birthed in the hospital. I had the perfect birth that I knew I would have, regardless of my size.
Then when I fell pregnant with DD3 I just knew I would cop it all again and I did. That Ob had moved on to another hospital but I copped it over my weight and because I had birthed so many babies. In the end, DD3 was born safely and beautifully at home with no complications to her or me. AND, after she was born we went to hospital in a REGULAR ambulance with no issues at all!!!!
Stick to your guns and stay empowered. You have the right to refuse anything they tell you to do. There is no reason why you should be under the care of the Ob now just because your BMI is on the higher side.
Good luck hun