Not unethical at all.

As already noted, you may not get the cheaper fare at the last minute. Apart from the ticket rules preventing this, there could be a waiting list.

So even if you release your reservation and immediately attempt to re-book, you might get nothing. It might even be impossible to get back your full-fare Y ticket. Best to attempt a new reservation before releasing the old one. (Some systems trap duplicates if they persist for too long.)