Your hotmail address is here to stay. And MSN will replace hotmail name with outlook for awhile and change it back to hotmail again.
It is a simple thing, people only use outlook at work, not at home. They will not use outlook at home, they use hotmail and those people will complain hotmail disappeared. So eventually they will change the button name back to hotmail on MSN again.
My suggestion, wait for 3 years before you switch to this lazy branding service. Outlook.com will only make more confusion to the users and be dropped again. It sounds negative, but, confusing is simply confusing, no matter how they want to spin it with statistic chart and politically correct reasoning. Human and users are largely not turing machines.