The shift coincides with the announcement that the first joint appearance is going to be in Philadelphia.

hArRis HaS tAnKeD iN tHe OdDs FoR tHe vP pIcK!1! - republicans celebrating

I’d still go with Kelly first, though Buttigieg is easily the best and wisest choice, if that makes sense. Regardless, he’s got many years ahead.
All being said though, I don’t think there are any bad choices - nothing that will hinder the momentum.