"How can I support her but at the same time make clear that I think she needs to just stick with a major? I want her to be happy and study what inspires her but the debt just makes me anxious and think about us struggling to pay our bills if we get married and start a family together. Sorry if this came off as a ramble, it's just been on my mind since she said she wants to switch majors. "

I agree with just about everything that has been posted here. The only thing I could add is there may come a time when the opportunity to have this conversation with her presents itself. We might call it "change talk", in other words listen to what she is saying when she's talking about this and look for opportunities to express your concerns as objectively as possible. Name the behaviors or issues rather than making it about her as a person, such as "I get concerned about our future together when I hear about this (or think about this, etc)."