In 2.9 you are able to change the name of a smart field but doing so would break the query. In 4.0 it was changed that you get an error when you try to change the name of the smart field which is great because then you don't break anything. However, it would make more sense to use the GUID in query instead of the name. In would be nice to be able to change the name in cases where there is a typo or the name is not descriptive enough.