https://twitter.com/Ququroon/status/713496244325253120
There's how it works.
They're unifying all the splashes that have two versions, picking the best of the two for each one and applying it everywhere.
They're also giving updated skin splashes to all reworked champions (Trundle, Karma, Nidalee, tristana) whose skins no longer look like their splash.
They're also keeping on with the base splash updates (that will likely happen to, say, Rammus at some point).
What they are CURRENTLY NOT DOING is updating skin splashes for champs that have not gotten a VU. So, while Rammus's base may change within a year or two, there's no current project about updating his skin splashes. Same for Corki I'd guess.
However, once they've done the currently planned VGUs, they'll do a few other VGUs (that will come with splash updates like Poppy's or Shen's) and then they MAY go back to doing VUs without the Gameplay part.