There could not come a time that for example multiverses just come to exist, as they would be contained within a definition that already exists. But if people want to change the 'current' definition for the Universe word, then will just have to introduce a new word that covers the definition, which already contains and entails Totality; Everything: All existing matter and space considered as a whole.
See, it is not 'the definition' that will have to change. It will be the word used that has 'that definition', which will have to be found or created, and changed.
Describing the Universe, Itself, is smaller or narrower terms means a new word will have to be used for the definition, all there is.