If your Apple ID (and hence iCloud login) is a non-Apple address then you can change it to another non-Apple address which has never been associated with an Apple ID. However if your ID is your @icloud.com (or @me.com) address then you cannot change it.
If you are able to change the ID it will not affect the account except that you will have to log out on all your devices and then log back in again, and if your have 'Find My iPhone/Mac/iPad' enabled on any of your devices you must turn if off before you start.
As to the actual email address, once you have created an iCloud account and chosen an @icloud.com address to go with it you can't change the address (short of creating an entirely new account).
However, all is not lost. You can add up to three 'email aliases' - these are additional addresses (not accounts) which deliver into the same inbox as the main account. (In fact it's a good idea to give out alias addresses, rather than the main address, because if they attract spam you can easily change them.) (New aliases can only be @icloud.com ones; @me.com addresses cannot now be created.)
You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account or move it to another account.
More information on aliases here: http://help.apple.com/icloud/#mm6b1a490a