How do I tell my current research advisor I want to work in a related field with another professor? I am currently an undergraduate student looking to work on a senior thesis. Over the course of the past summer and sporadically throughout the Fall semester, I worked with a professor in extremal graph theory. There has been some (from my point of view) subtext between us that I would continue this work into my senior year, though this was never explicitly said.

However, I want to pursue something more algebraic, and have been in contact throughout the Fall semester with a professor working in algebraic graph theory.

How do I tell my current research advisor I want to work in a related field with another professor?
Just tell him. Be polite. You're over-thinking this.