Did you receive something like this?
**You are NOT being asked to choose a single username to access each of them. Your Riot Account will NOT be consolidating all your existing accounts.** Your Riot Account is not comparable to Battle.NET where you have the option to connect to multiple League of Legends accounts from a single log-in. **Your current NA and EUW accounts will always remain separate.** What's changing is that your new Riot Account will be able to log-in on both NA and EUW - e.g. I will be able to take this account to NA. My current NA account will remain separate (and might as well be deleted hereafter).
**You need to update EACH account to be separately unique.**
**DO NOT START WITH YOUR MAIN ACCOUNT - you will be told that the username already exists. It does - by one of your alternate accounts.**
**You need to change-away each of the other conflicting accounts first. Once there are no conflicts - the prompt to change your username for your main account will disappear and you're done.**
Unfortunately, if (after renaming all your other accounts) you're still prompted to rename your main account - that means another email address owns that username. If you also own that email address/username - you will need to contact support. The system is reserving usernames on a per-email-address basis. As you do not have the option to change your username if there's no conflict, there's no way for you to rename your "old" account yourself if it's hogging your username (e.g. if your first account was on EUNE but then you created a new account on EUW that became your main, but you used two different email addresses, EUNE's email address would have dibs on the name and EUW would be prompted to change, even if you then created NA, BR, TK and PBE accounts on the second email address - they would all need changing). Support will walk you through the steps of proving you own both email addresses and swapping the names for you.
I imagine this PSA is too late for some users - you were told that your username was already taken and have picked a new one for your main account. (Especially if your browser automatically logged you in to your main account.) Don't worry though - support will also swap your account names if you made a mistake.
It would be helpful if the email explicitly said "REDACTED1 is available for you to claim, but you must first remove other accounts associated with that username." - but it doesn't. It isn't clear from the email if you're the reason that you cannot use your own username.