OP's question is about skipping a segment. Skip a segment and the remaining segments are cancelled. Whether there is a refund depends on the terms of the fare bucket, but likely answer is that the segments have no value and you buy a new ticket at walk-up fares.

As to changes to drop a segment, you can again refer to the terms of the fare bucket to let you know what, if any those are.

Bottom line: It's going to cost you. Don't wait. If you want to make a change, make it now.