I'm defending my PhD thesis early next year but have not broken the news to my advisor about my medicine plans. She and my committee will be surprised and probably dismayed that I'm not trying to become a professor like them. I wasn't going to tell her until I aced my upcoming MCAT in about a month. I need to ace my MCAT because my uGPA is very, very low.
You and I could not be in a more similar situation. I finally said F**k it and told my advisor as well as the rest of my thesis committee members. In the end, I really don't care what they think of me and my plans, and they were actually quite encouraging in their remarks. It only took me 4 years to realize the numerical modeling of the erosion of coastlines is mind numbingly boring, and that I would rather gouge my eyes out than look at one more partial differential equation. I am pretty sure this did end up screwing me out of funding for the semester, though as I did not get a TA position. I have never not been given one when I needed it in the past. YAY debt to add to medical school debt! My advice... tell them and anyone else who asks! I felt an enormous weight lifted off my shoulders when I did.