I'm blown away by the artwork for these cards, it's a shame that Celesteela, Pheromosa, and Blacephalon got the shaft though. I'm very excited to add the magnet line to my collection either way.
I can guarantee it doesn't work like Zygarde or Wishiwashi. Both of those Pokemon change forms through an ability, and Let's Go doesn't have abilities. It must be a completely different type of form/evolution, maybe like the Reveal Glass, or something based on how many Meltan you've caught?
Prerelease promos are always done by the same artist who made the original art. The Altaria have different artists, so it'll likely be a blister promo.