You craft torches, take down the lanterns and replace them with torches. I fail to see any logic that would think that replacing all current torches with lanterns would be a bad thing. There's no way I'd want to go around relighting all the torches I already have in place (has to be in the hundreds) after the change, and if for some reason you do want torches, replacing the lanterns with torches would be no different to going around and relighting all those torches.
I'd prefer they remain as torches not for any aesthetic reason, but because to me it feels like cheating.
Torches are cheap and easy to make, both coal and wood being common, but if lanterns involve more resources (such as iron, as has been mooted) then if I were "gifted" vast numbers of lanterns that replaced the many torches scattered about my world, then its as if I've been given (say) a big pile of iron, or glass, or whatever they're made from, which I feel unbalances the game as I haven't actually mined all those resources myself.
When Pumpkin Patch does come out I'll try it in my existing world and see how it feels, but if it ends up feeling unbalanced I suspect I'll just start a new world, which is a shame because I have invested quite a lot into my current world
I think it's a gift for all our support and encouragement through the development of minecraft, he WANTS us to have that unfair advantage :3 (By he I mean Notch). Also, I doubt lanterns would be made of iron. It would completely put a different look to the game that would make me RAGE QUIT. Plus, iron isn't plentiful enough even for a lighting tool that lights it so brightly.
Copy current save. Run update so it changes all torches to lantern. Paste old save inside lantern save. Voila.
Not necessary and unlikely to work. Notch will likely just be changing the torch block to be treated as a lantern block because that's far less hassle than trying to add versioning to maps where it doesn't currently exist, and less work than what would be a global search-and-replace.
MCEdit on the other hand will merrily let you select vast areas of land and say "replace all those old torches with new torches" so if you actually still wanted torches it'd take all of five minutes to do.
As to the iron thing, I have a ton of iron stashed away. It's common enough that I doubt it's really going to be a problem for anyone if one glass, one iron, and one coal makes four lanterns in line with the current recipes.