My first thought when seeing Katie as Fantine was "Isn't she too young? She's like 20-23" Then I realised that she's 28, and that made me feel old af. Still in denial that I'm also in my late twenties.
Since Marius & Éponine are portrayed to be closer/better friends in the musical than the book, it's a change that makes sense. Éponine very much believes Marius to be above her, so it's fitting she would address him more formally than him to her. That's my view on it anyways
I find it interesting that Adam Bayjou is still factory foreman/member of Patron-Minette. He's been in the show since like 2014/2015. That's a long time in the same show playing ensemble roles (though he's also alternate Valjean).
We still only have confirmed Enjolras, Courfeyrac, Grantaire (and Marius) of the Amis on IMDB, and with filming of the barricade underway it seems we won't get more Amis than that. Which sucks. A little less excited about the series now, to be honest.