I don't like that idea about deleting old world...

I think best way would be add multiworld support so anyone could go to old world and use new world also

there are Multiverse 2 plugin, I think it's best for multiwords, not sure if it works with
1.2 and maybe there are alternatives just need to do some research
I can say Multiverse is awesome I tried it some time ago

another way would be change spawn location to area that haven't been generated on map, so everyone could still go back to old houses etc...