Welcome to FT!

By policy, UA does not overbook their premium cabin*. Currently, UA is only offering a single seat for sale in F on UA6271. If they sell it, the inventory will zero out completely, and anyone else who books onto that flight will have to purchase Y or take another flight.

The only reason you'd get downgraded on this flight is if there's a broken seat or if a Federal Air Marshal happens to be assigned to that flight. However, UA does a pretty good job of selling out the front cabin, so there's a decent chance you'd be first in line for a downgrade anyway, no matter what flight you're on. Furthermore, there don't appear to be any other E-175s that can get you to LAS on that day, so if you change, you wouldn't get to try it out.

Long story short, I don't see any advantage to changing your flight that would be worth the cost to make the change. If you do somehow get downgraded -- again, extremely unlikely -- you can either accept the downgrade and get a partial refund of your miles plus a travel certificate, or you can request to be reseated in F on another UA flight and still get a travel certificate for your trouble. There's a thread on this: https://www.flyertalk.com/forum/unit...ation-etc.html

Good luck!

* On three-class planes, UA will oversell J, but only if there are seats remaining in F; then, they will op-up some passengers from J to F. They will not intentionally create a situation where they need to downgrade anyone.