Yes, you should cancel the return before booking the alternative. Overlapping reservations can be a problem and risk of cancellation.
With most domestic fares, RT is two one-way fares so there should be no fare change. There will be no fee to cancel.

It will require the PNR to be split, so you will be on separate PNRs on the outbound.
Are you checking bags? With some effort, you should still be able to get your traveling companions, bag waiver. But not on the return (and no E+ access).