If you have a purchased C+ fare, fare difference can potentially be around $520 each-way (roundtrip C+ fares start at $910 each-way vs. $1430 each-way for PS G fares). You don't have to upfare in both directions (you chose PS in one direction and C+ in the other and still qualify for the cheaper roundtrip fares). The advantage of an upfare to PS, instead of an online seat upgrade, is that you can potentially apply a Mileage Upgrade Award and get a D1 upgrade. You can generally upfare online by going through "Change Flights" option.