Seems like the OP is talking about Eurobonus.

Unless there is current availability for CPH-LAX I wouldn't cancel and expect the seat to go back in award inventory. Expecting this can easily lead to disappointment (and frustration that the confirmed C/J class seat to LAX is gone).

Most likely you will not be able to modify the existing EB award without cancelling CPH-LAX unless you find an agent who really knows what he/she is doing and is prepared to go the extra mile for you. It is much easier for the agent to cancel and book CDG-LAX from scratch. I suggest to give it a shot, but you should make it very clear that agent shouldn't cancel the CPH-LAX segment.