Keltham will actually cast Glimpse of Beyond - unwarned, but Carissa has probably guessed it's coming, since she's the one who suggested that - verify he can see the correct original people beneath them.

And then immediately turn around and look the other way once he's done that, and ask how long the spell needs to stay up.  If it's not long, they can turn back now.

Actually!  Keltham will cast Glimpse of Beyond - unwarned, but Carissa has probably guessed it's coming, since she's the one who suggested that - and verify he can see the correct original people beneath them.

And then Keltham will immediately turn around and look the other way once he's done that, and ask how long the spell needs to stay up.  If it's not long, they can change back now.