Big Thunder Mountain is a classic that needs to be preserved! Changing it to Big Grizzly Mountain would be like changing one of the classic Haunted Mansion attractions to Mystic Manor (as impressive as that ride is otherwise).
While Big Grizzly Mtn looks like a lot of fun, I wouldn't trade out BTMRR for it. Big Thunder is classic and fun. Out of control and enjoyable. If anything, add a western expansion section to Animal Kingdom and throw it in there (although it is kind of similar to Expedition Everest).
Get rid of some of the Meet and Greets the Tiki Hut Room, and some of the other boring waste of time rides and keep both! Don't get rid of an awesome ride for no reason.