The biggest ones that need updates are the extremely old legendary skins that were released at the time where legendary skins were the equivalent of epic skins now, similar to how ultimate skins at the time (Pulsefire Ezreal and Spirit Guard Udyr) are now more like today's legendary skins.
The Magnificent Twisted Fate and Annie In Wonderland are most notable because they don't even have new voice lines, whereas every other old legendary skin (Brolaf, Corporate Mundo, Demonblade Tryndamere etc.) all at least have that (correct me if I'm missing any old legendaries without new voicelines, but I think those are the only two). At the time, it was a big deal for a skin to have new effects and a model overhaul, but now, that's a typical epic skin. Either those two should get an RP drop to at most 1350 or a total overhaul to be true legendary skins.
Legendary skins that at least have new voicelines, like the three I mentioned above, would probably be more in line to get Gatekeeper Galio/Nunu Bot-esque overhauls.