We booked a 3 leg flight on Aeromexico (in business class) through American Express and paid with points for $$.

6/12 LAX > BJX (Leon, Mexico) (Aeromexico 2703)
6/16 BJX - MEX - OAX (Oaxaca) (Aeromexico 0135 / 2036)
6/21 OAX - MEX - LAX (Aeromexico 099 / 0648)

Then we heard that Oaxaca is not very friendly to dogs, so are looking to modify the flight and skip Oaxaca this trip.

Our ideal itinerary would be.

6/12 LAX > BJX (Leon, Mexico)
6/19 BJX > MEX
6/21 MEX > LAX

So, my main question is how it might be possible to cancel the Oaxaca legs of the flight but maintain the existing seats on our flight home 6/21 MEX to LAX which apparently has jumped up in price.
American Express was unable to accomplish this from their side, but thought AEROMEXICO could do this, however the last agent I spoke with there, was not able to do that without rebooking

Does anyone have a suggestion for how to cancel any of these segments without cancelling our return flight or do i just need to keep an eye on prices and hope they drop the level that we purchased the tickets with.

Last night American Express had almost rebooked me (for all the flights i needed, w/ a credit of $239 remaining) but I waited until this morning to contact Aeromexico so I could use the credit on the pet fees! But, they told me the changes that I want to make would result in a total charge of $650 (including the $250 change / rebooking fee)

Any suggestions are appreciated.
Thank you in advance.