Why can I not delete my ppa and/or change my username

Users with a PPA are not currently able to change their username.

If you have (ever) published packages to your PPA, then as I understand it, you can't rename your account (as all the packages exist on disk in a directory that uses your current username) and we don't currently support deleting PPAs.

There is a bug to make this possible (https://bugs.edge.launchpad.net/soyuz/+bug/392887/comments/7)

In the mean time, a work-around might be to create the new account with your desired username, and then merge the old account into the new one as described here: https://help.launchpad.net/YourAccount/Merging. Your karma may be temporarily set to 0 though it should be corrected within a day. Your "member since" date will be the creation date of the new account.