Not got good advice on the general question, but very much disagree with:
Math is instrumental and something you will pick up as you go along.
You need to self motivate yourself strongly to pick up new mathematical fields to the level where you can contribute. I'd make sure you had math courses in formal logic, whatever else you had; that would make it easier to learn the relevant stuff later. It also helps with thinking about these issues.
I studied computer science and I wish I had studied math; I think I'd find computer science much easier to "pick up as I go along".
Currently I am planning to start a mathematics degree when I enter university, however my interest has shifted largely to computational neuroscience and related fields, so I'm now planning to switch to an AI degree when I go to study. Having said that, MIRI has always posed interesting problems to me, and I have entertained the thought of trying to do some work for MIRI before. And so my question boils down to this: Would there be any problem with taking the AI degree if I ever wanted to try my hand at doing some math for MIRI? Is a maths degree essential or would an AI degree with a good grasp on mathematics related to MIRI work just as well? Any thoughts or musings would be appreciated :)