How easy or hard is exploring for an undeclared student?

My son was accepted to UCR as undeclared in CHASS. I think that was his second choice, ultimately for a major in Accounting or Finance. But he’s also interested in sciences and math, and maybe engineering. How hard or easy is it to declare a major in one of the other colleges down the road? What is the process? Is there good advising to help an undeclared student take the right classes to allow the most opportunities in this regard? Thank you in advance for your replies.