Yes, because the redo of Adventureland is focused on the tropics but with more modern IPs rather than the period-piece 1930's vibe that is currently in place for all Adventurelands. Would UP work in Disneyland's Adventureland, even if there was room, not really. But the proposed new AL, sure. Paradise Falls fits a jungle-themed adventure attraction.
And I don't view Grizzly Peak as expendable more so than I view Pirates or Space Mountain expendable. I've stated that its the one area of DCA that is still firmly in the California theme and doesn't fit a park with random IPs. So if they want to change DCA to a mark with a bunch of random IPs and no real regard for identity, then yeah they change Grizzly Peak into something fitting that IP-centered park identity. If they want to keep Grizzly as is, then they need to honor the spirit of the land/park and Up wouldn't compliment that, unless, as stated, it was in Pixar Pier similar to how Pinocchio and Toad work across the way from one another under the umbrella of a vintage European courtyard consisting of carnival-style ride throughs of Disney animated classics.
The story of Up is about an old man who always wished to visit a specific place in South America. He then flies his house to this place in South America and spends the rest of the film having adventures in that South American location, where he meets Kevin and Doug.
Does Grizzly Peak look like South America? Do you want to retheme Grizzly Peak to Paradise Falls and ditch the California Redwood theme to Grand Californian? Or did you want to swap out the one key location and motivation of the film with another random natural area that has nothing to do with Paradise Falls? Like Carl just randomly flies and lands in the Redwoods for an adventure after caring so much about South America. Then why theme it to Up? That's like making a Coco ride where are with Miguel's family in LA. Sure, it has their characters, but moving the story and not featuring the one built up location would be pretty silly. No land of the dead, no Mexico, just the characters hanging around a different location that is shoehorned into DCA?