Yes, we change Dorico’s file format frequently. Any time we add even a single new option, that is a file format change.
But you don’t need to worry about changes in file format. Dorico 1.0 can open files saved in Dorico 4.0, but obviously there will be a lot of data loss as Dorico 1.0 doesn’t know anything about (to name just a few things) chord symbols, chord diagrams, repeat endings, unpitched percussion kits, cues, condensing, markers and timecode, etc. etc.