Ugh, there were so many missed opportunities... instead of retconning a crucial part of the lore, they could've...:
A) Rework summoners. What is a summoner, what do they do, who are they, where do they come from, what is their relation to the IoW and the champions, etc.
B) Retcon summoners, but keep IoW, possibly rework it. I'm pretty sure IoW could keep its position as a kind of a '' problem solver '' without summoners or a menacing force that is trying to use champions for their own gain. There's also the possibility of reworking IoW to a more '' entertainment '' related thing, like Pokemon tournaments. This way IoW can exists, without sidetracking from the IMPORTANT, RELEVANT story-telling. This would also mean League of Legends THE GAME is yet again part of the lore, yet it's not a BIG PART of it, but you can use it to write some lore IF YOU SO CHOOSE TO.
C.1) Destroy summoners, keep the IoW. If summoners are a big deal, you can just make champions go against them and rebel. Combine this with a possible new champion, new game mode and you have a ridiculously cool and fun event planned, that is also important lore-wise.
For instance, champions are getting fed up with summoners getting all the credit, wealth and recognition (comparable to proplayers and League celebrities, that use League of Legends as a platform to jumpstart their career as a gamer/entertainer) and champions feel as if they are nothing but play things in a game called League of Legends. Include a new champion that likes to cause drama, war and riots, and possibly manipulate peoples feelings, thoughts and subconscious, and there you have it. Champions vs Summoners. And how about a newISH gamemode? Champions start breaking the rules of League matches (Awesome Buff of Awesomely Awesome Buffing, robbing the merchant, start with level 18?) and are up against the best AI that the Summoners have developed (Doom Bots) and there you have a ridiculously stupid game mode that is sure to be intense and fun.
At the end of event, champions have won, and IoW is remade into a neutral '' problem solver '' as it should've been in the first place. Whether it continues to be a entertainment central or a political arena, that's really up to writers.
You could also add an item commemorating Summoners and the war that happened between champions and summoners.
C.2) Destroy summoners, destroy the IoW. Same thing as above, but IoW will crumble as well. It's better than retconning everything...
D) Rework summoners and IoW to a clearly neutral force that doesn't interfere with conflicts, and doesn't force or recruit anyone to join their battlefields. They can still maintain their status as a conflict solver, even if they don't push their agenda to the high heavens that might make IoW and summoners seem too powerful.
But whatever, retconning it is, instead of replacing less important lore (than IoW and summoners), or giving something the end it deserves.