United will normally downgrade you if 1st is full. But, they will not automatically upgrade you to 1st if coach is full.
If I am flying united and it is iffy for coach I will list for 1st and let the gate agent know I would like coach if there is a seat-I have to pay extra for flying 1st class on United. But, best thing is to sign up for 1st and if you do not want to pay the extra fee let the gate agent know to downgrade you if there are seats.

No way of knowing where you fall (I think the new phone app shows this but I do not have that type of phone). Once the merger is complete the new united system should show you where you fall.

Tony