Here's something Marco requested some time ago.
Alright, so let's say you're playing "Patch 1", and you find this shiny OP item. (It isn't even that overpowered, but whatever, that's for example purposes only)

But this overpowered item in fact never meant to be that strong. Marco is just retarded and mistakenly typed 2500 instead of 250. Oh noes!
Now Marco fixes this in "Patch 2", and you're thinking: Hehe, fool, I kept it in my stash.
Now Marco could trick you and make the item level's requirement 666. But that renders the item useless, and hey, you did find the item in the first place right? And to say it had only one chance in 1,348,102 to drop. That would be quite a shame to lose the item while it was the mod maker that did a mistake.
But that's no longer a problem. Now legacies are updated in real-time (assuming there is an updated version and it's defined in uniqueitems.txt). So when you update to "Patch 2" and create a game, your old overpowered legacy gets deleted, and gets replaced by a shiny new updated and nerfed version. Yay!

tl;dr: legacies are auto-updated in real-time