I actually think his plan is a little bit more elaborate than just to show-off. If you look at the
full preview, Doom actually wanted to claim Excelsior and Teddy's position as King Of Space. It seems that when he realized the sword only responds to Teddy, he decided to body-swap for that reason. That makes me wonder if he might actually try to stay on Teddy's body. And if Excelsior will be able to tell the difference between their minds/souls. Or if Billy will violently kick his ass out of his husband's body with his magic, as he should!
Yeah, that was pretty cool. If he actually manages to stay on Teddy's body for a while, he's gonna have so much fun with his shapeshifting. lol