If Detective Beckett was a guy, the show would be different, at least a little - because the show is centered on Rick Castle. But casewise, would anything be very different between Kate and Chuck Beckett, assuming their backstory (mom assassinated, dad on the wagon) was the same?
If Detective Ryan was a woman, would the show be very different? Or if Alexis was a guy?
(respectively: probably not much; and likely as much as with Beckett - since Alexis tends to be a mirror to show Castle what he's doing, or to help solve a case)
* = (if you know the thread, good; if not, no worries)