Interesting thought and one I like the idea of. They should not jump a year, otherwise my elementary school year 5 son will go into year 6 without having covered the work in year 5. Even though it may be desirable to align the starting time of year with much of the rest of the world (but not all the world, I know of one country that starts its academic year in January), I'm not sure whether it's desirable to align the ages. Britain is a bit of an outlier with school starting at 5, which I think is too young, and I think that Japan has the starting age right.
As well as the old guard, there may be cultural resistance to not having entrance ceremonies coinciding with cherry blossom, even if it no longer happens for most of Japan because of global heating.