League of Legends is having you change your username and it's based on the new IPs on the way by Riot Games. Riot Games surprised everyone with a massive push toward the end of 2019 announcing multiple projects in the works, plus even more down the line.
The League of Legends launcher will inevitably turn into a Blizzard-style launcher. Battle.net allows fans of every Blizzard game to come to one place to either play their game or connect to the live service. Riot Games will make the same thing for their games and is asking players to change their username.
League of Legends Change Username: Riot Games Having Players Change Username Ahead of New IPs
Riot Games shared their thoughts on the matter, "One area for improvement we’ve identified is our old account system, which wasn’t as prepared for us to transform into a multi-game developer as we’d like. Once we dug around, we found that we flat-out couldn’t launch more games unless we made some major adjustments. Because of this, we’re making an adjustment to our account system that might require you to make some changes to the name you use to log in."
If you can't log in but don't have an email, you can use this tool to see if your username is on the list or been impacted by the changes. To change your name is rather simple, either click the link sent to you via email or click the prompt when you attempt to log-in, in the League of Legends portal. Both will take you to a place to change your username. If you've been prompted to do so, there is no way around it.