Informing your advisor that you are changing disciplines As a sort of follow up to a previous question, what is advice you would give for telling my MSci advisor I am switching disciplines (pure maths to statistics) for doctoral work? I will presumably ask him for a letter of recommendation. I do not want to come across as ungrateful for his work with me, but the direction of my current research with him has come to bore me. How do I tell him that is a major reason I am switching disciplines? Do I even need to disclose such? I am closing in on completing my work with him, so anything dealing with changing advisors right now is not an option.
Directly.

"I've decided to pursue a PhD in statistics."

Then after the inevitable discussion:

"Can you write me a strong letter of recommendation?"