The University of Chicago and Columbia both have probability classes that you can take without being accepted into their schools. The University of Chicago's is online and Columbia's is in person in NYC. You could probably register for it in the summer, take temp jobs for half year, study up, and then apply. Also, I would find a C++ course that you could take along with it. These schools may also have that. Unfortunately, I do not have the specific information on the classes, but if you Google them, or speak with their admissions, then hopefully you can find the specific classes and how to register.

What really matters is if you have an ability in math and programming, so it's a gamble as you do not know yet. If you cannot do it, then it does become difficult to get into these programs. My recommendation is that you take the GRE and if you cannot do very well on the Math GRE, then maybe it's not the right fit for you.

I actually decided to double major in math between sophomore and junior year, so I understand how you can realize these things later than expected. I was an economics major initially then the mathematical side really interested me. Good luck!!