So they called in Caitlyn's 
voice actor to make her new Legendary Pulsefire skin, and i cant for the life of me understand why didnt they also re-update her base voice lines sort of like they did with lux, to give her interactions or updated lore quotes...
Just why not? You have the actor there... seems like such a missed opportunity.
This isnt just with Caitlyn ive seen this multiple times.
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Sometimes when they do legendary skins, it's not the same voice actor half the time. Plus they recently started to do this. The Lux VO update wasn't in the same patch as her Elements skin. They waited a patch before adding it in. So we'll have to wait a bit to see if they updated hers. I'm also surprise about PF Ezreal's VO being updated too.
They did this already with Lux, and Lux's updated VO wasn't released for many patches after Elementalist Lux was.
They very well might have gotten a Cait VO, we just haven't seen yet.
Are we just ignoring the possibility that they probably DID update base Cait's VO & it hasn't been released yet? Maybe it's not ready, maybe they're waiting for a certain date/event, maybe it is coming with the next patch & it wasn't a part of today's PBE update. Why are we assuming they didn't if we have no way of knowing? Of course, there's the possibility they didn't update Cait's VO. But can we at least not talk like it's a fact?
Even with Lux, Elem Lux came out in November & her VO update came out last month. That's a several month difference & hell, that was them testing the water. With Maokai, they probably had his VA in to work on some other project too (whether it be a new Maokai skin, auditioning out for a new champ or rework or some other shit)
Didn't the Lux VO come out several patches after Elementalist Lux was released?
Like what many has said it might come to us on a later patch but I think it could also be that they want to fully VU her. So they might just wait to update it then.
Assuming they in fact did not record a new VO possible reasons would be money constraints and time constraints aka the VA schedule.
I also wish they did this for Thresh. This is something they've had two opportunities to do now with Dark Star Thresh and the Dark Star Thresh Announcer.
Uh... remember Elementalist 
? It took some time for Riot to release the VO for the default skin.
What make you believe that they are not working on Cait VO as we speak?.
Caitlyn kinda requires a visual update. In terms of visuals, she's kind of an old champion and in terms of the Piltover Champs, she doesn't really fit in (Ezreal is up to debate as well).
EDIT: Basically what Neaman said
They probably did, or already have, and have just kept them until Caitlyn does eventually receive a VOU.
Same occurred with Lux and her Elementalist Skin. They got the VA to record some lines for Lux too, and released the VOU shortly after.
Well you know when they did Lux's for her Ultimate skin it was done in 2016 and the audio for her base wasn't released till later in 2017. So they may have had her do some audio for the base as well, but we just don't know yet. They don't appear to release these things all at the same time.
Got any spare change to pay the actor for additional work?
Well on the one hand they might add it later,like they did with Lux. On the other hand it is possible that there were problems with the original voice actor in other languages.(But then again,Cait doesn't have much of a VO to begin with,so it shouldn't be impossible to replace it in those languages)
Movement, aa, spell animations is not changed, so they will just modificate the voice sound,they wont put any other lines. Pulsefire is not a Legendary skin, so it will cost 1350RP, being an Epic one.
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