Can someone change PhD topic within a department after admission? Suppose that someone applies for a PhD in the US with publications in topic X, and hence the whole SOP and
the target group both revolve around X. They get admitted, and realise soon after entering that there is another topic they would love to work on, albeit it is worked on by a different group.
Is it possible to switch? Or are they supposed to work on X with the same group during the whole PhD?
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.