Users buy an account, automatically have user name change to a certain case number,and other things

Now I'm designing a drupal site for a guy and after weeks of work and frustration I have a prototype up for him, unfortunately he wants some changes that I dont know how to do :(

I'm using UberCart right now so that:
1. users can register an account with us
2. then go to a form that they fill out, and submit it
3. then it takes down their credit card details
4. then it posts it up to the site until an admin acn look at it and make it live...

now what he wants to do is change around the order and have it so that:

1. Users buy an account,
2. automatically have user name change to a certain "case number"
3. fill in the application form

now I cant figure out how to make it so that I can do that and especially to have it so that account can only post up one form per "case number"

he also wants to get rid of the "my Account" menu when you log in

if anyone could help me out or just point me in the right direction that would be greatly appreciated thx :D