When they introduced the sidekicks on the campaign they said that were not final and would probably change. If she (and maybe Rekke??) will be a full companion at some point, it wouldn't be better if they are just added in the expansion and on the base game are replaced with other sidekicks? that would solve the weirdness of being "upgraded" and instead of replacing a sidekick to add a full companion there would be the sidekicks and the added full companion. I mean, i don't see a problem with this solution, please Mr. Obsidian, do this.
Here is the thing, people are getting too excited over it.
In the original Eternity Kickstarter Forton was advertised as one of the original companions. He is on the key art, the box art, the mousepad, etc etc. What happened to him? Name change, slight art change, and ended up as an expansion character. Let's not even get into what they did to poor poor Cadegund.
Just because someone is being touted as a character or sidekick in the kickstarter doesn't mean they will actually make the game, or that their appearance in the game won't be different.
I am confident Obsidian knows how dumb it would be to make a side kick magically become a full companion in the expansion. If they choose to make a currently advertised sidekick into a full on companion in the expansion I have no doubts that they will vanish from the sidekick list.