maybe I can help a little, though I chose to notify my care provider (a cnm practice) by phone that I would no longer be using them.
you don't specify your reasons for switching, or what alternative you are choosing (CNM, CPM, lay midwife, UC?) so I will make my suggestions general:
dear dr. ____,
I am writing to inform you that I have chosen other arrangements for my care in the remainder of my pregnancy, and for my birth. I believe that my needs are no longer compatible with your practice, and I have another provider better suited to my needs. Thank you for the care you have provided thus far. I am also requesting a copy of my prenatal records, that I might pass them along to my new provider. Please notify me of the fee, if any, for obtaining the copy of my records.
OK, hopefully that is helpful. in all honesty, I would not go into much detail about your new choice, esp. if it involves a midwife who is questionably legal in your state (I am coming from the view of an aspiring midwife, and also someone who had a lay midwife attend her HB) because it could cause problems for her if the OB decided to be vindictive. Unfortunately in some states the health dept. and local drs. are prosecuting midwives, and using whatever ammunition they can get. I would stick to the facts, though I think constructive criticism is a great idea. if you are leaving because you recieved less than ideal care, tell the OB. make sure to get your records. they may charge a small fee to copy, but it should not be unreasonable.
and blessings on your journey!