As the other have said Unexplored will be fine as the new content will generate in when you explore it. Failing that there is a Mod you can run that will reset Biomes you have explored so when you go back to them you will have the new stuff pop up as if you had never been their so all is well :)
I should clarify the Upgrade world mod just needs to be run once and it resets the world you do not need to run modded after that just once