He haven't ruled out presidential bid, so the door is still open, providing Kentucky law is changed in time.
Kentucky law, isn't going to be changed, since the GOP doesn't control both houses of the legislature. However, Paul could still try to convince the state GOP to allocate their delegates via caucuses, rendering the presidential primary in KY meaningless.
But the problem is the general, no?
If he actually wins the presidential nomination, then he's obviously going to drop out of his Senate race.
He just wants the ability to run for both in the
primary, but the state legislature isn't going to help him out there, so he has to resort to these shenanigans with the state party switching to a caucus.