They can change it at any time. Look at all of the well engrained rules that have changed in the very recent past, like starting extra innings with a runner on second or the pitch clock or the three batter minimum for RPs. The owners don’t want to shorten the season because they are missing out on revenue.I was talking to my Sox-fan buddy today and we were wondering if the Cubs and Sox had been better during the first few years of Baseball's expanded playoff format from 1969-1982 and there'd been several bad weather postponements due to trying to play baseball in Chicago in October, whether baseball would have been forced to alter their season schedule to end earlier. But because the expanded playoffs were ingrained into the baseball culture by the time the two Chicago teams ever got any good, Baseball doesn't do anything about the bad weather.
And it wasn't just the Chicago teams... Detroit, Milwaukee, Minnesota (still in an outdoor stadium back then) and Cleveland were all pretty awful throughout the 70s. So baseball never had to confront the fact that they had a playoff format basically predicated on hoping the weather in the upper Midwest won't be too awful in mid-October.