This question comes up quite a bit - It's something we would love to do! I'd love to go in and update old sounds and particles and all that too. The reality is that right now, we just don't have the bandwidth to accomplish something like that in audio, and a few other areas too. I'm in audio, so I've gone through the process a few times.
To revamp the VO of a champ, we'd need to write a new script, make sure it still feels like the champ, find a record date with a voice actor, get a studio and record, edit recordings, process them, send the way we processed them out to localization, implement them in game and make sure everything works the way we intend it to, make sure everything gets localized in a ton of languages, and more. There's a ton of people that are involved in something like this. Doing the VO for a new champ or Legendary skin actually takes a few months!
So this is why I love the Champ-Up team. Any time they do a full rework they do all of this. It's actually a ton of fun watch them search for the perfect actor for a champ and then test out a ton of different lines.