I think we can divide the GU in two parts
the first one is more a scope update( like the roster class updates) good example is sejuani, her spells changed but not the idea.
the passiv is still a tank passiv, the W is still a are dmg spell with the bola, the E is still a active spell for "freezed" enemy and the ult is still a good engage, just the there is now more powerplay: Passiv is now just 2seks in the beginn of the fight activ, W is a skillshot now, E interacts with melee allys and has a stun that deals percent dmg and the ult is now for 1 enemy and the rest is zone dmg.
the second one is actual a big gameplay update: a basic example s rek, she was a tank, now she should go more for a fighter than just a full tank, But a bigger example is Aatrox he will get a GU that change all his abilitys completly without looking at his kit now.
for Splashart updateS: this is a potentional idea, but will not be so popular, becasue the splash art team has to work on VGU, new champs and new skins . So they haven't much time to focused on Splasharts that needs updates on their. I understand this, becasue no champs get balanced or more popular becasue the splashart changed
for VG. The main reasons are: wishes from the Community, potentional , Gameplay, Visual , Lore or just a modernisation . The Community wanted that Urgot gets a Rework, also his gameplay and Visual was kinda old, but they hadn't a good idea, so it take some time until they start working on him. Swain is now the next one: His gameplay is very unhealthy, either you are undieable or you are useless, so no skillexpression. His potentional is big as THE leader of Noxus army. Now he is just a person with some crows, but not a great tactician like the lore lore said. So they want to give him the next VGU.
evelynn and WW are both champions that have a good idea , but don't fit in the lore of Runterra or has no skill expressions, but the kit was good so it is a modernisation.
His passiv is the same, but now the effect is based on the hp of WW to make it stronger when he is low and weaker when he is full.
His Q is still a percent dmg with a heal, but now he has the choice if he wants to dash behind him or just stay on the same play
His new W is a mixture of his old W (simple attack speed buff) and E (vision on low enemys) , now he is able to chase with the passive low enemys and get attack speed and movement speed, but is not able to get vision, just the position and a active that marks the nearest champion, but don'T care if he is low or not
His E is now a good tank tool and CC , ww had just the ult as a cc that makes it hard to fight someone when the enemy was fast, now he can fear enemys and get damage reduction
The Ult is now a skillshot that alows you to jump on enemys that are not visible just with your passiv
So all in all the modernisations are good,becasue the spirit is the same than before , just more fun to play and to play agaisnt
The same will happen with eve, the idea of perma-stealth will be the same, but she will get tools to do something in her "shadowform"