Thanks, Matt. Indeed sharing a library folder led to some problems when going from 16 to 17, so I'll be sure to avoid doing that same mistake again
bunglemebaby: Yes, since MC 18 converts the library to a format not supported by MC 17, MC 17 will no longer be able to access the library if you move the new library created by MC 18 to the location where your MC 17 library is. So if using both you have to maintain separate libraries in separate folders and import new stuff and make changes to both separately.