Doctor Won't Provide "Predictable Outcome"
No surgeon can predict an exact outcome or guarantee a specific result. It is unrealistic to think they could. The best we can do as surgeons is to give guidance in your decisions based on the years of experience we have, and in dealing with the type of surgery you require. When dealing with corrective surgery, it is even more difficult to predict outcome. Constricted or tubular breasts are challenging in that there are usually many things occuring to cause the cone shape. In addition to the breast implants, we try to release the breast so that it can fill to the shape of the implant as opposed to the implant simply filling the space/shape which your breast has formed. When my patients have concerns prior to surgery, I invite them to come back to see me so we can discuss the procedure so you feel more comfortable with proceeding. While your surgeon probably has good reasons for changing to a different implant, it's important for you to understand why, and what it means to your outcome. Try to understand why he cannot make any guarantees, but at the same time, open the dialogue with him/her so you can feel good about the decisions you make.
Concerns about moving ahead with breast surgery for constricted breasts...
I am sorry to hear about the stressful situation. Ultimately, it will be very important that you feel comfortable that your plastic surgeon has significant/demonstrable experience achieving the types of outcomes you will be pleased with. Viewing lots of before/after photographs of your plastic surgeon's work and/or talking to previous patients will help alleviate anxiety.
If you still remain concerned, my best suggestion: communicate your concerns again with your plastic surgeon's office; hopefully, they will comply with your request… Best wishes.