Originally posted by Latcady:

OK! I spoke to Smiffie and he say's that the previous info I had is still correct... the general goal they have is to avoid forcing people to restart worlds when big content drops like H&H come along, but it is possible as the game continues to grow that older worlds will get harder & harder to update with new stuff fully. So you may get some things from a patch on your old world, but not everything because some things require a "clean" environment to generate in.

So... you won't totally lose access to old worlds (at least for now - this might change in future depending what ends up getting changed/added) - but they may not be as fully populated with new content as a new world would be.