There are "new mechanics and items to discover and play with," according to Thorson, and you'll have to complete Chapter 8 to unlock these levels. There are no B- or C-Side variants to this batch of stages, though.
You might have to wait a little longer to try the levels on Xbox One. "We apologize, but coordinating this patch across all consoles was challenging for our tiny studio," Thorson wrote in a blog post. "If it doesn't make the 9th, it will release soon after." Meanwhile, now that the team has finished work on Chapter 9, production on delayed limited-edition physical copies of Celeste will start soon.
Alongside the DLC release date, Thorson revealed he and his colleagues opened a new studio called Extremely OK Games. "On TowerFall, my collaborators took a larger role than I anticipated, and on Celeste, it was obvious that calling ourselves Matt Makes Games had become silly," he said. The team is now working on a game with the codename EXOK1.