Originally Posted by
hopping
Is it possible to get to A from E8 without having to clear customs and immigration again?
You don't clear customs or immigration to exit the US. As already noted, you will have to reclear security to get from your incoming UA gate to A, there is no airside connection.
United will NOT check your bag through unless you are on a single ticket. They also will not check through for a connection more than 12 hours.