Change major to computer science before beginning freshman year?

Hi!
I am going to be a freshman at UMass Amherst next year (I took a gap year after I got accepted in high school) and I was wondering how hard will it be to change my major to computer science before beginning freshman year? I got accepted into Isenberg, but I changed my mind and would like to switch. I know that you can email UMass to change your major but I would like to know what are the chances that I will be able to switch since computer science is a very competitive major at UMass.
Thank you!