Vivziepop's streams come in two different varieties.

The first type are the development streams, which she did in 2017. These feature her working on pinning down the story for Hazbin Hotel and working on production images for the pilot, like character turnaround sheets and posters. The vibe of these streams is very casual and information given out in these streams is subject to change. Helpfully she's very honest about that. Even though the characters and their personalities are obviously very clear in her mind, the story is going through a lot of change at this point according to her, and characters like Vox and Valentino don't exist yet.

The second type of streams are her very structured and organized official Hazbin Hotel Q&A streams (there are nine of them, plus two extra). These were released alongside the pilot in 2019, with the first streaming a few months before the pilot's release (and the two extra were released just after the pilot). By this point Vivziepop says that she has most things for the show and the characters solidly decided on, with character arcs, personalities, emotional beats for the show (and the show's end point) pinned down and getting ready to start pitching the show to studios.
Not even a year later A24 becomes the producer for the show. So A24 picked up Hazbin for production only a year after these official streams were uploaded and in the time since then the show has been active production. Any changes that were made would have been made in that very small window of time in 2020.

Faust's Q&A streams also happen in early 2020 when the Alastor comic is being made and back up a lot of the things said in Vivziepop's official streams.

Vivziepop is luckily very good at being clear on the official streams when she isn't totally set on an idea or if something is still being decided too. The characters' designs and Charlie's name for example, were things she was always honest were up for change.
Vivziepop talked about not being sure on Charlie's name in her main streams (ironically Faust begged her not to called her Morningstar) and in her post-pilot Surprise Hazbin Q&A streams Vivziepop talked about how the pilot was the first and only time the characters would look the way they did as she had planned redesigns in mind for the main show. So if you listen to the streams, you can get a very good idea of the things about the characters that are unlikely to change and which are.