Let's consider Fiddlesticks.
Under his release lore, he was some mysterious creature brought forth by a novice summoner, who Fiddlesticks killed. No means of communication was met with a response from Fiddlesticks other than to attempt to kill any who entered the room he was first summoned to. Nothing could be discovered of his origins either. The solution was to keep the room sealed.
New lore....well, you do have the Institute issue, it doesn't exist. But now instead of a simple boogeyman mysterious figure they HAD to give some story that he was some poor farmer that was lynched and he was raised because evil magic crows and shit and now he's angry dead spirit wanting blood and revenge.
It is an example of a complete delete of lore (they could have tweaked old story to just happen in Noxus or somewhere on Shadow Isles) and also removing mystery and mystique by getting so specific with the origin.