I'm not sure that you can. that firstname.lastname@example.org is called the NAI, and is only used internally for access to Sprint network services.
I've never had to use that for anything. It's stored in the phone and you never have to give it out to anyone.
Access to Sprint.com, as you found, is controlled by what you enter on the Sprint.com website.
I just bought my phone and service this past weekend. They used my full name plus a number for @sprintpcs.com. I do not want that as my username. Why not the email@example.com? Something less obvious or invasive.
"Go to Sprint.com and sign in with your username and password" doesn't help when you were never told your username or password or asked to set one up. So without more lucid instructions, I signed up for Sprint.com with a username I'd prefer and set up that account searching for my answers. This didn't help, so I crossed my fingers and added my phone to that username giving me access to my bill etc., but I cannot find the option to change my username (aside from changing the Sprint.com username which is not what I want to change). One support document says to go to "Administator" something, but that does not show up anywhere.
Can I get some clear instructions on this? And, can I just change that email address/username and have it only work for service, as the current one appears to do?