To be precise, the COC only provide for a refund of the difference. Thus, if OP's selected seat cost $100, he is entitled to a refund of $30.

While not expressly stated, it is likely that UA would also reassign OP to an E- seat and refund the entire E+ fee.

My suggestion is not to request any refund at this time. If a better seat, more to your liking, opens up between now and departure, grab it, If it does not, you can deal with the refund post flight. Bear in mind that E+ seats may open up between now and the flight as people cancel, no show, or are upgraded.