I just went through a migration for my newly created account and it all went well. Except now, my profile link contains the randomly generated ID as a user parameter instead of the actual username itself.
My previous profile was accessible via : https://open.spotify.com/user/myoldusername
But now my new profile is displayed as : https://open.spotify.com/user/afay5aqk2r6vfl5gqvggbmiha
I know it may sound silly, but when it comes to online sharing, a link like this is simply not aesthetically pleasing. And it's actually pretty useful to be able to access your profile by typing out the first part of the URL followed by your actual login username.
Hopefully you can look into it :)