Each time is an opportunity to update old/outdated Voice Overs. I would have loved to see some new voice lines and special interactions for Heimerdinger, especially toward Ziggs. Stop missing these opportunities and sell us some dreams.
Champions who need Ezreal sized VGU: 
I was very sad when PF 
came and we didnt get a VO update. the PF VO is super good
It did happen with Lux soon after Elementalist Lux came out, but not with anyone else.
Although it is a nice thought, it's not as simple as that.
Some actors have different contracting and licensing terms that may have changed in between voicing the original champion and the new skin. Sometimes, scheduling conflicts happen and the actor can only be in studio for a little bit (although I know that for League most of the audio is recorded in the actor's own studio, but scheduling is still an issue)
As well as this, localization is a concern. If the Japanese voice actress can't make it, but the others can, are you gonna change the entire base VO or are you just gonna give the new skin a new voice? Most fans would dislike it if the changes weren't consistent. There are some exceptions, but typically Riot likes to keep voices consistent across all skins and languages (although some companies, like HiRez (Smite), don't really care).
So VO updates like Lux's only happen if all the elements come together.
