You may standby for First and will get it as long as A inventory is available. But, of course, as you know, if F is full, then you could always standby for coach.

As to changing your connecting city: it is generally allowed, but always check the permitted routing of your specific fare.

Years ago, you were not allowed to standby for differerent connecting cities, but that rule has been changed. If your fare allows a routing thru ORD, and an agent says "no" then politely ask them to check the rules again. It changed several years ago, but plenty of agents still think you may not change connecting cities when standing by. They are wrong, unless your particular fare routing doesn't include your alternate connecting city.

Good luck.