Is the $420 rate a fully-refundable "Y" class fare tkt, if so then the answer is no CF applies and you would be able to get a refund.

The dreaded "N" class (at that rate I highly doubt this applies) does not allow any changes, so if you miss your flight...you are SOL.

If the tkt is anything but those two classes, then you answered your own question only partially correct. The $100 CF applies, but there is no refund on the difference.

If the tkt is not fully-refundable, I'd recommend that you purchase the new lower priced tkt and hold on to the unused tkt for another time. Just be sure to retain the original tkt number in order to reapply it. You will have up to 365 days (one year) from the original travel date to apply it toward future travel--keep in mind any fare difference and the CF. As long as you apply the value for a tkt just under the $420 range you only have to worry about the CF.

Check with reservations to confirm this. YMMV. Sorry for not responding earlier, I wanted to ask at the station just to get consensus before responding.