I called AeroMexico today and the total fee for the changes I needed ended up being $129 per ticket - which is much better than the $650 from earlier in the week.

I also learned that all my AeroMexico flights will not accept a pet under the seat in premiere class! The AeroMexico website lists that is only available in certain premiere classes, and i guess these planes don't support it.
They were also are unable to change the class of my tickets to coach, where i could have brought a pet.

Huge bummer, but probably best for our dog to stay home at this point.