If you pay the $300 cancellation fee, the rest will be refunded to your credit card--no need to deal with travel credit or trying to arbitrage the credit through a cheap domestic flight. This is a refundable fare (albeit with a penalty for refunding).
There should be a separate "Changes" section to the fare rules that will also spell out a change fee, but typically on those fares changes would be a $250 fee ... so if your plans change your choices are to refund for $300 or change for $250. All of the fees are waived for qualifying schedule changes--I think that the carriage of contract specifies that the schedule change must change your arrival or departure time by more than 2 hours to qualify (or changes carriers, such as switching from mainline to UX), but I haven't looked at it in a while.
The "waived for schedule changes" is standard language on all fares. What you're paying the extra $150 for is the ability to get a refund to the original form of payment rather than ending up with a travel credit that expires after 1 year.