If they don't change the skills themselves, but only the levels they are taught at, it might seem like it only impacts instructors. And that's mostly true. I bet even instructors probably don't need to retrain - they just need to look at the new list of skills for each level.
But not quite. If a student has passed through a level, he/she might assume he/she is ready for the next level. But now that might not be true, because an assumed skill may not have been taught to them yet. If you take the next class in the very next session, you can trust most experienced instructors to try to compensate for the missed skills. But after a while, they won't. If you miss a session or two, you may have to repeat a level, maybe more than one level. People will be annoyed.
It's a little like when Arthur Murray [I think ?] ballroom dance classes changed names and content (many years ago), and all the students who had taken their lessons found they were no longer qualified for a specific next step - that there WAS no clear next step. I never took their lessons - they were very expensive and more or less aimed at the pro and semi-pro level. But I knew students who felt cheated. There were a LOT of complaints.