I can't imagine the differences in versions being that drastic, so it's probably going to be easy to adjust overarching changes to the 4 major variants. I doubt they started these versions without thinking about the potential difficulties of overall plaza changes.
Besides, it's not like these are constantly on rotation. The team will have multiple months to get the next version ready for use, so they don't need to make sure that all four variants are always up to date.
As for the Halloween plaza, it seems that some additional effects are used as well as plenty of additional decorations; I'd guess that this is the primary reason for your framerate issues. The classic plaza is a lot cleaner, so I wouldn't worry about the effect some minor changes to the usual version would have on framerate.