Change of interest right before qualifying exam
Given that you're in the US, my advice to you is slow down.  You don't need to pass your oral exams in your first year, and studying for and passing an exam in a subject that you intend to drop completely is not a good use of your (or anyone's) time.  Moreover, you can stop working with (or not start working with) your advisor without jeopardizing your future in the program, so there is absolutely no reason not to be forthcoming about your concerns.

I suggest talking to your current faculty advisor and devising a program of study which allows you to explore ML while not quitting symplectic topology "cold turkey".  As a first year student you can afford to spend at least a year taking courses trying to figure out what you want to study and building up background in that area.  In fact, it is very unusual for a first year US math PhD student to know exactly what they want to study in the future: that kind of decision usually lies at least a year away.  

It sounds like you thought you did and that you have some pride in your own advanced standing.  That's understandable, but the thought of going ahead with an examination in a subject that you plan to drop soon thereafter and with an advisor that you plan to drop immediately thereafter is really not a rational, wise or kind way to proceed.  I think you need to take some time to gain a better perspective.  Sharing your proclivities and your plans with your peers and mentors is a good way to do so, but again: take it slow.