In Yahoo Mail, it's possible to create "disposable" email addresses consisting of two parts, an arbitrary "base name" and a "keyword".
Once set, is it possible to delete and start over with a different base name?
I have been trying to change it too. It seems that you can't. The UI confused me and I ended up having a really stupid basename. See this link for more info.
Important - Once you create a base name, it's locked into your account and cannot be edited or removed. If you'd like a different base name, you can create a new Yahoo Mail account and add 1 base name to it.
You may try contacting their "tech support" but being a free account, I think they'll just ignore us.