@Memor-X can't test the update right now but apparently:
- old planets terrain did not change. But biomes... could. So the plain near a lake you built your base on will still be a plain near a lake... but it could have turned into something more funny (for Dwarf Fortress definition of funny - enjoy your wormy neighbor)
- star systems will have more planets than before, so you could find new planets in system you already visited. Those will be far more diverse than the "regular" planet.