I recently went through this experience with a wonderful teacher. I had been with him for almost 10 years and pretty much exhausted his taiji curriculum. (He did not teach taiji weapons or freestyle push hands.) Also life is short (maybe I was feeling more mortal post 9/11)and I wanted to experience some other views. I told him up front that I really enjoyed his program, but that I wanted to incorporate some other things. His response was that he felt badly that he did not have more information to give in taiji (he teaches Shaolin as well), but he understood. We're still friendly and I talk with him from time to time about techniques. I think a student owes the teacher the respect of being up front and polite. If the teacher is a good person (someone you would recommend to others), then he or she will care about you and respect your views on what is best for you. If the teacher is not as mature as we should expect, and reacts badly, then it's the teacher's problem, not yours. I agree with the preceding post that you should not burn any bridges or be rude or disrespectful. I would also add (somewhat tangentially) that my teacher, who'd had his share of problems with one of HIS teachers, always told me to separate the messenger from the message. Put another way, you can learn a lot from people even if they aren't perfect.
By the way, my decision was very tough for me, because I felt like I was losing a good friend whom I'd spent time with every week for years. I still miss the classes, but I don't regret the decision, because it's fun to learn something new (as long as you aren't jumping from form to form without learning at all).