The SVS documentation parenthetical [line 292] "(an Expanded Value Set is a set of concept representations that were in effect at a specific time for a particular version of a Value Set definition. The Value Set (definition) and the Expanded Value Set concepts are similar to the programming concepts of Class and Instance of Class.)" simply notes that the combination of a value set definition version and a date can yield an expansion. It does not specify that the expansion identifier will change very time the definition is changed. While the identifiers in VSAC are consistent with date information, they essentially provide a clue to the first date that object (such as an expansion set) was valid. As was noted in Steve Emrick's comment, VSAC expansion identifiers will not change if a definition is updated but the expansion member list based on that definition did not change. This is a benefit to the user so that they can always compare a version they have to the current version and determine if the membership of the value set had changed, independent of any definition change. Keep in mind that a definition identifier only tracks the details of how a value set was constructed. The OID is linked to purpose/meaning, and the expansion version is how you tract what members were valid on a particular date. So no, the expansion identifier is not always going to look like a date that is later than the definition identifier.