Evelynn for an example, she looks very different in the splash compare to her model in game, is this a hint on what her rework might look like?
http://ddragon.leagueoflegends.com/cdn/img/champion/splash/Evelynn_0.jpg
They said that when they do Evelynn's VGU, they won't do much changes to her appearance. They would try to match her with the current splash art, but with a few tweaks to her design.
I am pretty sure Warwick and Fiddlesticks splashes wont change, especially Fiddlesticks which wears the green cinematic clothes in his new splash
More likely they'll do pull a "Shen" and tweak her new splash a bit with her remake though I don't see them changing much as it appears the newer splashes for Warwick and Evelyn are the directions in which Riot would like to have them designed after.
Most likely not. They didn't even start with the VGU in most cases(only Ryze,Yorick and maybe Warwick are currently in the works),so it is unlikely that they settled on a design already.
For example Shen and Tristana both got new splash arts before their VGUs,but their updated design looked different than their splash arts(though at least shen didn't look that different),so their splash arts had to be updated again.
Most likely not. They didn't even start with the VGU in most cases(only Ryze,Yorick and maybe Warwick are currently in the works),so it is unlikely that they settled on a design already.
For example Shen and Tristana both got new splash arts before their VGUs,but their updated design looked different than their splash arts(though at least shen didn't look that different),so their splash arts had to be updated again.
You're right I do remember when people was talking about how Tristana splash didn't match her ingame, but wasn't her rework was on its way to being released and Riot said the splash was just a stand in until she was fully released which didn't take long for them to do?
But champs like Yorick(now he is getting updated), Poppy(Finally got updated), and Taric(Got updated), didn't get any updated classic arts after they was reworked and they had to wait for years using their old outdated splashes.
But Riot did say Evelynn, Urgot, and a few other champs maybe getting their reworks early next year, and you could be right, updated splash arts may not be set in stone because ideas change over time.
They said that when they do Evelynn's VGU, they won't do much changes to her appearance. They would try to match her with the current splash art, but with a few tweaks to her design.
Can you provide link or quote? You maybe right, Riot may go with that design in the splash but ideas change over time, we'll see how much they change early next year.
most likely
Looking back to the 
rework as a reference, It's safe to assume they will likely change the splash art, but only enough to match the new model.
