For instance....does this dial work with current, active supers? I mean, can you dial Superman and become Superman? What happens to Clark if you do?
Zoat already answered the in story answer, but how the dial work depends on iteration in the comics, if you're curious.
In the original series, it just works, no explanation given. He once became Plastic Man, and his girlfriend once became Supergirl. The Dial has Interlac writing on it, which is the language of the Book of Oa and the Legion of Superheroes, which suggests alien or time travel origins though.
In the second series starring Chris and Vicki the transformations from the imagination of a classmate. Kind of a nod to the "the transformations are based on reader submissions," it ends with Robby giving his dial to that kid, who since he makes up the transformations was able to choose what he transformed into. He chose to become the Shapeshifter or something, a guy who had the power to transform into any superhero he could imagine.
In Superboy and the Ravers, the dials summoned forms are from the user's subconscious. Hero afraid for his friends, he became a healer. Wanting magical help from Sebastian Faust, he transformed into Sebastian Faust. Vicki, who was transforming into monsters and not superheroines because she felt like a monster and not a superheroine.
Post flashpoint in the China Miéville work Dial H they work by interdimensional shenanigans, allowing the user to transform into an existing superhero somewhere in the multiverse. So theoretically those dials could allow you to transform into any Superman including the local one. If the dial works right, nothing happens to the person being copied. That person losing their powers is a known glitch that can happen though. The main character freaked out when he became the Flash, worried that he had depowered the Flash and might have gotten people killed by so doing.
So that's three ways the dials work that Zoat can choose from, presuming he doesn't just make something up he likes better.
