For example, this happens from time to time everytine a new patch comes out

It means that when the new patch drops the item will change, either for the better or for the worse. Just like characters get "buffed" or "nerfed" Some items that are retroactive will do the same come the new patch. So if it is retroactive you dont have to reobtain the legendary to obtain its new stats you just simply wait for it to change.

This post was edited by HereComesTheKraken on Aug 7 2015 01:45pm

so all set items are retroactive?

Correct, set items are typically retroactive, so a 2.2 set piece will have the new set bonuses once the patch hits.

and if a item isnt retroactive, then tat means u need to find the new item to gain the changed/new effect?

Yes