To change a non-refundable ticket, you'll pay the difference if the fare is now higher plus a $100 change fee. The fare difference is apparently $331 of the $441 total. Sounds like it'll be cheaper to just buy the one-way ticket and contribute the cost of the unused portion of the original ticket to the SUSA fund (Save US Airways). Unfortunately, that's not a charitable organization so you'll get no tax deduction....
Jim
Added:
Not knowing when you're traveling, if you have time you may want to just keep checking fares . I see that US has one way fares of $156/$159 for Monday so maybe something cheaper will show up for your travel day.