Originally posted by Zathabar:

^as above plus (and I realise I am starting to sound like I have shares in this or something! I havn't) The upgrade world Mod can be run once to reset explored areas then uninstalled so you can enjoy the new in a previously developed world without loosing any buildings either (unless you opt to!)

there is also a console command to load the content into the game but both methods come with the risk of the world seed dumping the new content into bases but no one should have built there any way so it should be 100% safe