The new items will only spawn in unexplored areas of the map in the biomes they're from. So if you have explored all of your Plains biomes, then you definitely will need to start a new world. If you haven't explored much you're probably fine to continue in the world you're in.

I'd recommend not getting TO attached to any map till the game is in retail release as future content additions will likely have the same requirement.

I'm fine with losing hundreds of hours of progress because to me this is all part of the process and I'm holding off on my real passion project building till retail release & just having fun till then. But I know some people are not prepared to let go of their maps and it's going to hurt! So just keep that in mind, have fun & play, but be ready to lose it all lol