If you are a shared customer, we could actually shift your account account to a plan under your reseller plan. This is an HSphere feature which we haven't tried yet. But if it works, then you'd have to do nothing. If it doesn't, you'd have to reupload and transfer all your files to an account within the reseller package as if you were with a new host.
Yes, the system would generate instant access domains with your service domain name - d13133.x23.yourservicedoname.com. You'd have to create name servers for your service domain (with your registrar) and point them to the IPs given to you. Then you'd have to point your service domain to those name servers. After a 48 hour propgation period, you could access your control panel - cp.yourservicedomain.com, servers - win1, win2, win3, mssql1, web, mysql, pgsql.yourservicedomain.com and your name servers. Once your service domain is setup, you could create user accounts and deploy your corporate site in one of them.
You could customize headers, the login page area, colors, signup text, etc. See the Reseller guide at hsphere.com