You can use either your @me.com or @icloud.com address. They are interchangeable. Anywhere you can log in with your @me.com address you can now login with your @icloud.com address.
You are not swapping from @me.com to @icloud.com. It is still the same account, with all the same info and purchases. It is simply an additional email address you can use in addition to your existing @me.com address, if you want to.