What happens if my PhD interest has changed after a year?
Since you've been admitted to what's presumably either a single institution, or a small number of institutions, it's probably more useful to try to figure out how things are handled there than what is typical. I would suggest scouring department websites, and maybe contacting your graduate coordinator, or graduate student representatives first.

That said, if you care for generalities, here are a couple. First thing to note is that the stated research interest is unlikely to be binding. If you can find an advisor to work with, that's all that matters, regardless of field. Changing interests is fairly common, but perhaps not between two extremes. However, the selection of admitted students tends to depend on the available funding, which varies with field. Thus, being able to switch to e.g. astrophysics isn't necessarily easy in practice, particularly if it's a small department. (Now, if you come in with your own funding you can do whatever you want, at least as far as the department is concerned. Of course, whoever granted you that funding might think differently...)

Second, there tends to be a number of mandatory courses for all students, and also field-dependent mandatory courses, and elective ones. (Obviously, biophysics and high energy require very different courses.) Due to these electives, and being able to take classes in different orders, the first two years would not be the same for everyone, but time isn't necessarily wasted if you change interests either. Further, a lot of students find their advisor during/after the first year, and would start their research during the summer semester and second year.

Yet, as I said, that's just a quick rundown of the typical situation. You'd most likely be better served looking up the rules and options for your specific situation.