Bam is markedly bigger and more athletic than Thompson. That's not an upside scenario.
It's a realistic upside scenario. Maybe you didnt realize, but Thompson was the starting center on the most recent Championship team. I guess you could go into fantasy land upside and say he develops a reliable jumper, offensive moves, and ability to pass, but I'd be surprised. His best role would be something along the lines of Tristan Thompson's. A team where they wont ask him to do much offensively other than putbacks and dump offs and he can focus on imposing his strength and athleticism on the boards. If he goes to a team that tries to develop him outside of that I think he will fail. He will have to pick up the intensity too because he wont be able to rely on just being bigger than everyone else anymore.