To change the ticket to another travel date/flight you owe the change fee and the fare difference, if the same class is not available. If you cancel the ticket prior to departure, then you get a one year travel voucher/credit in that passenger's name but you will also be assessed the change fee.
Same day changes to the ticket have different rules and depends on your status with United, or lack thereof.
If you fly United frequently and/or plan to us them in the future, I would buy P, and accept that it will cost $450, if something happens. A lot better than the 6k or so,