I have a large explored area in a pre "Pretty Scary Update" world. Including several swamp areas. Will witches spawn in those areas? If so, is there a stand alone tool (like this one for slimes: http://mcslimes.appspot.com/ ) or similar for finding witch spawn areas?
Are there any swamp huts there?
If yes, then witches will spawn there.
If not, you could go to single-player, select the world, and then press re-generate to re-generate the world again (as a different save). This will use the same seed, so the swamp huts will be there in the re-generated world. Your inventory and world will still be in your old save.
Your third option is to port them over using MCEdit, a third-party program used to customize your world.
These responses are incorrect, they will spawn in pre pretty scary update worlds. dinner_bone confirmed this somewhere. I used the tool "amidst" which tells you where witch huts should be, and built a witch farm there.
edit: here is my source
and here is AMIDST, perfect for finding witch spawning locations in old worlds. link