Was thinking about the Kanto bird forms earlier - I know myself went straight to thinking they were Galarian forms, but I wonder if they might just be forms they can swap between like the Therian forms. I got a shiny Zapdos from the Wonder Box, so kinda hoping I can use it for the new form.
I don't know what to expect anymore, since they love being unpredictable.
The most obvious seems to be that they're Galarian forms, and so, you wouldn't be able to change forms (like any other regional form). The strange part is that we've never had regional variants for legendary Pokémon, so I don't know what would be the explanation but I guess it'll have to do something with that tree.
There's a slight possibility though, that they're not Articuno, Zapdos and Moltres, but regional evolutions instead. Or even, completely new Pokémon altogether, like the new Regi's we've seen. Tbh, I wouldn't rule out anything at this point.