Most likely if there is unused data in the show file left over from previous picture legends it shouldn't cause issues while running the show but might make saving and loading a little slower. There is a possible way to fix the show file but it isn't the most straightforward so make sure you keep a backup of the show that you can go back to.
The show file is stored as a zip file which contains a binary version of the show file that is quick to load but is not compatible between software versions and several XML files which are what is used when loading the show into a new version of software. What you can do is open the show file in a zip tool of your choice (e.g.
7-Zip), delete the binary version of the show (
Show.d4b) and then load the updated show file. The console will automatically switch to loading the XML version when it cannot find the binary version of the show and will load in the same way as it would if the show was from an older version of software. When it does this there is a certain amount of clean up that happens which should hopefully reduce the size of the show file the next time you save it.