Evolving a NoSQL database schema regularly involves migrating datasets into new schema versions. NoSQL databases store datasets in different heterogeneity levels (HCs) that can be characterized by their degree of regularity and cardinality of various entity types. In this article, we present the semantics of NoSQL evolution operations and their corresponding data migration operations while distinguishing different NoSQL HCs. One use-case of NoSQL evolution operations is improvement of actuality and completeness of data which is especially relevant in terms of the ever-expanding volume of data.