Originally Posted by RealHJ

Redeposit and rebook, so the $150 redeposit fee will apply. No way around it, other than being DM or PM.

Many DL agents will insist on there being current availability for all flights, i.e. to completely cancel and refund your original ticket and book a new one from the ground up. Don't let them do that (if needed, HUACA). Insist that they keep the existing PNR and just change the flight(s) being changed, but let others remain as they are. Still will be $150 fee, but your PNR will stay the same and the other flights seat assignments will remain valid.