I decided to veer off and tried doing a computer science degree and an astronomy degree my sophomore year, and I do regret the stress and lack of sleep it cost me. I eventually did settle for just an astronomy degree.
That being said, one of the computer sci courses I took was a 2 semester course that was programming for Computer Science Majors, not the regular C++, or Fortran Intro classes. It was much more beneficial, as in the latter intro classes they were fairly basic and consisted of weeks of simplistic scripts, i.e. Hello World. The Programming for CS Majors was more intense, and more beneficial in the long run - even though it was in Java. Once you pick up a programming language, the general way to setup complex programs, and a little info on time/memory allocation issues you're good to go in any language.
So if UM has a similar type of intro course specifically for CS majors, that would be most beneficial. Instead of going for the major, it would be better served to devote your remaining time into research, and studying for the physics gre well in advance.
And Go Gophers! - I'm a former Minnesotan