Didn't only one of them drink the elixir?
Ostensibly, but do you really think that the author's going to stop at just one of them switching at chapter six? Like, if the changed happened in ch1, then I'd believe it was just the one. But here the author has a whole roulette set-up, and we've spent multiple chapters wondering who was going to switch. (first) What's going to happen to all that tension if this is it?
Plus, we've gotten several hints at the others potentially being in the process of switching as well, so we can't be entirely sure that "only one of them drinking the elixir" is an entirely sound argument against multiple being affected. Maybe the elixir was a placebo all along, maybe it's like ice-nine and converts nearby liquids into itself, maybe whoever was setting up the cups intentionally put some in all of them?
And even if the elixir did only affect one of them, that's just this time. Maybe the fact that Goh didn't turn out how they expected will prompt them to try again using some leftovers of the drug or whatever?