Yes they will be. I mean the 6* that is.
They wont be "upgraded" per say - instead a completely new version of the said rarity will be maid as a standard Gacha character.
So for all reasons and purposes its a completely new character, just the name stays the same and her current low tier looks will be followed for the new artwork, but with a different usually more detailed spin on the whole thing (in case its not specific theme like Kimono, Beach, etc).
It is a "pseudo" upgrade if you want to think about it like that. The old low tier version still remains low tier and useless but if you get the new 5/6* version than you can treat it as the low tier one albeit actualy usefull one, even though technically its a completely different girl that just shares the same name looks and personality.
Having all 3 of the last low tier girl "upgrades" I must say, they are by no point any weaker than what you get from other standard non-event gacha girls.