automatic check-in tool
 

Is there any tool that automatically check you in 24 hours before the flight is scheduled for departure?

From what I understand, the sooner you check-in there is slightly a higher chance of getting upgraded.

automatic check-in tool
 

UA has an option for automatic checkin on the return leg. Not sure about other airlines

I would not think so and I would even think it could be detrimental... But that's just my own opinion based on some limited experience...

How could it be detrimental to check in early?

Just using common logic, I would think that if you're checking early and flight is oversold (that's the only reason for op-ups nowadays and I believe that you were thinking about those) then people who are checking later have better chance of being bumped up because they may not have seats for them while you already have the seat... But I might be completely wrong...