I mean, I get that they need to have unique account names moving forward. But someone please explain to me why MY account name needs to change when it's existed since October 2009? It would be physically impossible for someone to have an account name matching mine that predates mine, so why am I the one that needs to change it?
Because Riot, for various reasons, is not giving the oldest account the name by default. There are multiple reasons for this that you’d know had you read the FAQ.
It doesn’t matter when your account was created or the others. What matters is when you change the name, does someone else have the new name you want - even if that’s the same name.
someone did the same username as you, but on a different server
