It will make me somewhat sad to lose my castle that I built, but it's time to play in Biomes, in hell, and...
if Survival MP has functioning damage, my SP worlds won't see any play because my friends and I will be playing on a private MP server and it will be too awesome for words, torches that burn out not withstanding!
I would like to continue my old saves, but they are mainly just my first attempts at feeling the world, understanding how it works and trying out ideas. New maps will give me the option to start 'smart' from scratch and also play with new goodies.
But if damage works in Multiplayer, most likely my friend and I will play and share home with some creepers; oh the laughs/emotional fits we will have.
I'll restart. Day before reset = me deleting all of my worlds and embracing the new.
It's almost like converting into a religion, but without the ********.
I don't want to start a religious debate, or add to one, or whatever. But I do indeed think that the scientific answers are best.
On topic, I'd delete a world or two and play on 'em, and keep old files.