The disposable email set up is terrible on almost every level. Yahoo appears to place no value or importance on email addresses in this modern age. They appear to be stuck in the nineties. Just completely incompetent and out of touch company. Obviously when setting up a disposable email and a mistake is made or change is needed one should be able to easily make changes to the "base name". The arrogance, incompetence and failure of tech company's in staggering.
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.
Additional info here: https://help.yahoo.com/kb/SLN28338.html