No one here can answer that, but I think it would be especially punitive to cast you from the kingdom after they already accepted you, just because you don't want to pursue the PhD.
How can you get loans and pay a bill now if you have no tuition? How will you not signal what you are doing if you tell them to keep their money? Pick your poison, because, if they are going to retroactively want reimbursement, you are not going to be able to retroactively borrow that from the feds, and will have to find that money elsewhere (savings, parents or private loans).
I don't see how waiting to tell them until after classes begin is not going to be worse for all involved (i.e., making it too late for them to replace you, if that's what they want to do). If they want to throw you out, why will they not be able to in late August, versus now? I'd think they'd be more inclined to do that, out of spite, if you wait until they cannot find another PhD candidate to take your place.
JMHO, but I would not screw around with this, given the stakes.