Can someone change PhD topic within a department after admission?
There might be exceptions, but generally in the US it is possible to change, though doing so might cost you time. The exceptions would be if you were being paid by a particular group (through a grant, say) and then, while it would still be possible, you would have to arrange funding.
But, for the most part, you are accepted into a department, not a specific group within it and you can probably change within that department. You would need to find an advisor to accept you, of course.
Most students start doctoral studies with only a BS/BA and little immediate research focus.