Write special interactions for charters who deosnt have them
to
“You are playing with powers beyond your comprehension, child”
Kills enemy 
“I finally did what he should done all those years ago”
2 Comments
While it's all well and good to write new lines for outdated champions (and I'm sure some lore Rioters have done that as a pet project), there's still the matter of finding a decent voice actor for the champion. Riot (logically) doesn't like to do that for only a few lines, and therefore has to rewrite the champion's voice script.
Generally speaking, Rito doesn't do this unless they're updating other parts of a champion (Swain, Irelia, Akali, etc.) or already hired the VA for a legendary/ultimate skin (Lux).