You don't say what kind of phone you have. If you have an iPhone, then I recommend that you use the Passbook feature. Once you check in and view the mobile boarding pass in the United app, there will be an icon to "Add to Passbook." If you choose that option, the boarding pass will be copied into the Passbook app, which runs independently from the United app. Once it's in Passbook, you can always view it there, independent of the United app.

Another thing you can do on the iPhone, which might also work on the Android, is to take a screenshot of the boarding pass. On iOS, just click the main button at the same time as the power button, and it will take a screenshot and put it in your Camera Roll. Then just display the picture later on.

In any of the above cases, if the boarding pass is no longer current, perhaps because of an upgrade, it should still get you through security and into the lounge (if you're eligible). At the gate, the scanner will beep to inform the agent that there's been an update, and at that time they'll tell you of your new seat assignment.