I'm a first-year pure math PhD student. I'm going to take an oral qualifying exam in a few months, and for this I need to find at least one person who agrees to be my adviser and one of the examiners in the qual. I'm interested in symplectic topology, and I have already talked with a professor of this area, whom I was going to ask for being my adviser. However, my interest in machine learning got revived, and I'm now very certain that I want to change my path.
Reapplying to CS PhD isn't really a good choice for me, since I didn't take any CS course in undergrad. However, I have studied this area by myself, and the process was very easy compared with studying pure math. I've audited topics courses in ML, from which I believe I don't have any lack of prerequisite for this subject, and that I'm ready for research once I finish reading relevant papers.
I'm aware that, once passing qual, I have freedom of changing the area to pursue. For example, a pure math PhD student having done a qual on symplectic topology can get a PhD by writing a thesis on computational complexity. While qual is not difficult for me, I have no idea how to actually proceed from now on. Should I tell the professor that I want to take the qual with him as my adviser and that I will not work on symplectic topology after that? (After that, I will find a co-adviser from CS department with whom I will work on thesis.) Or should I not tell him my intention until after the qual?