How hard is it to transfer from CS Engineering to CS at UMass Amherst?

I got accepted into UMass Amherst for my alternative major, which is CS Engineering. However, I really want to study in the CS program in the College of Information and Computer Sciences. This is mostly because my academic focus is Artificial Intelligence, and this is the reason I applied to UMass. How hard would it be to transfer from the Engineering school to the CICS school? How soon would I be able to transfer?

you would transfer as soon as you have met the requirements to switch majors. That means up to CS 187, which you would take first semester if you have AP credit in cs or second if you don’t. Specifics can be found here: https://www.cics.umass.edu/ugrad-education/major-change-cs.
If your competent, getting a B should be easy.