I think, if the change would be accepted, the rule won’t come into effect immediately. Like you mention, they will probably lay out a plan on how and when exactly these changes will apply.
I don’t think anyone, who’s already senior, will be forced back into junior ranks.
I am personally fine if it is accepted, equally fine if it’s not. I think for many current seniors it will be a big relief, because often due to insane competition within Russia, girls are forced to run a marathon with a speed they would run a sprint. So it will definitely give them a little break, which I think is a good thing. So I would expect for Daria, Kamila, Maiia, Anna, Aliona, Sasha, Anna F., Ksenia S. this would be a bit of a time, where they could slow down and not be threatened by the likes of Sofia Akatieva and Veronica Zhilina. However Samodelkina, Petrosian and Muravyova all would probably still manage to make it into seniors before changes will start applying.
I think the consequences on the junior ranks are even more interesting to me. The junior age category now will be 4 years, which is a lot of time. Will they keep it like that, or make another age category from 15 to 17?