honestly the old one is kind of ugly and isn't going to have much resale value. I would just "rip" it out... literally, with no regard to saving it... Can the insert be removed from the mantle beforehand? there should be a gas shut off valve somewhere behind it. just make sure you close that valve before disconnecting the gas line from the insert. Don't be scared if you hear a little hiss when you first break the connection in the line as there will still be a small amount of gas in there. Nothing to worry about tho. You can probably bet on having to repair the dry wall around it to some level of extent.