I asked this question once. To my knowledge, the only champion to get a full VO update is Cassiopeia.
A red responded to me, and explained to me what happens when they need to make a new VO. In the case of a VO update, they need to get in contact with the character's actor and work around their busy schedules. They pull the actor in, record lines, and then go to editing. Their sound designers do their work editing the dialogue, and in some champion's cases, they add effects; reverbs, echoes, that sort of thing. Then they implement them into the game, including movement quotes, attack quotes, item quotes, taunts, jokes, grunts, yelps, shouts, deathrattles, and other such things. The lines go through testing before being greenlighted.
Then, they do it again, and again, and again, and again, for every language that League is played in across the world.
In some cases, this might literally be impossible; certain actors might not be available anymore. For example, the VO for Twitch's VU is the same guy as his original, but when they did Omega Squad Teemo, they had to find a new actor because Teemo's original actor was no longer available (Something about being in a union). In this case, they have a conundrum - do they replace all of the champion's old lines with a new actor, or keep the VO that players have become attached to?
All of this is an very expensive endeavor. It's costly not only in terms of money, but in terms of time. It would delay VOs being produced in new champions or Legendary skins. The only case I know of where Riot did something like this is the original Shurima event, which brought new lines for Xerath, Sivir, and Renekton, and an entire new VO for Cassiopeia.