Lol, tf? Boruto can be retconned but Sarada can't? :lol Come on, Boruto didn't have a seal nor a doujutsu when Kishimoto wrote the movie and now in the manga adaptation he has it.
When did I say that? I said that the Anime isn't retconning anything on paper. The problem with the Boruto Movie, was that they tried to continue it on paper. So that's where the retcon shit came from. It doesn't work vice versa when there's already established content on paper. But IMO, people will gloss over the Manga's version of the Movie and use Kishimoto's.
In regards to Boruto's continuation, the first chapter of the Manga showed that he had a Dojutsu and a seal. The anime takes place before the Movie, so by that logic anything could of been fixed. Even Sarada having a teacher for the Academy arc, for argument sake.
But in regards to them trying to retcon already established canon like the Gaiden, Boruto Manga Rehash, and the Mitsuki Gaiden. Gonna be stupid move if Kishimoto tries to say it's "canon".
As you said, Kishimoto is supervising the anime so if they want to make changes in Gaiden, they'll ask him and he'll probably approve those changes.
We don't know what Kishimoto is doing, but he wants to make the Anime better. Although we have a list of the supervisors, and the biggest name is Ukyo. Despite this, Kishimoto stated himself that the first arc was canon.
So he has to do the same for the rest of the upcoming arcs, unless he out right says that the entire Anime is canon/official. Which he hasn't yet.
I'm not saying they'll throw in new random characters in Gaiden, I'm saying they can recreate the fighting scenes, such as making Sarada use new powers in them.
I don't think it would amount to much unless Kishimoto says something about it. But if they do, I would think they would follow the Manga's fighting sequences.
I don't see anything wrong with them.
Gaiden was written by Kishimoto in early 2015, just like the Boruto movie was written in 2015. The movie had changes/adaptations to fit whatever is going on with the plot now. Just like the movie, Gaiden will probably have it's adaptations to fit in the new plot as well.
I think the only reason why they tried to re-adapt the Movie was due to the Boruto Movie written by Kishimoto was more of a "finale" than a continuation, because they left no more plot points to discuss about the next generation. Meanwhile, in the Manga's version, they adapted new scenes which makes the story continue.
In regards to Naruto Gaiden, I don't see why would they need to change the story or anything of that nature to a Boruto rehash level.
For future references, I don't give a shit if the Boruto Movie rehash has Boruto learning Chidori for example. If Kishimoto doesn't state it's canon, it's not.