About units getting demoted after their banner

There is no such thing as far as I know, so you'll still get all the info from their respective hero pages. As for the upcoming children banner tho, it's safe to say that Rhajat will get demoted and in Dorcas and co. it's possible that none of them will get demoted (as was the case with brave heroes). The only possible demotion there could be Lute, but even she is kind of unique to just get demoted.