Add different iCloud email to existing AppleID
Even though the Apple support article referred to doesn't specificaly say this (I'm assuming it's because @icloud.com was only introduced recently and the article was never updated), what goes for @me.com and @mac.com goes for @icloud.com (they're all aliases for the same address).

ie, read: Some Apple IDs, like those ending in @mac.com, @me.com or @icloud.com, can't be changed.

So no, unfortunately you won't be able to change.

iCloud: About your icloud.com, me.com, and mac.com email addresses
http://support.apple.com/kb/HT5441