Can I have different version of data for the same element extant at the same time?

Yes, every NeTEx data element can be versioned, and multiple versions can coexist. Coherent sets of data are assembled for exchange using a ‘version frame’, which itself has a version and knows the version of the elements in it. There are specific types of version frame for different type of data that ar commonly exchanged together (SERVICE, TIMETABLE, FARE etc).