On its own it should only download and nothing else, if you actually went through some prompts then it can depend and download and then install after that all as one, or possibly prompt you for the actual install once it's done downloading.I'm on 7.1.2, but I did that straight from the latest iOS6
Where do you find the OS to delete? I've looked under Usage and don't see anything but apps.
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I think what happened though is that I said OK to the update notification when I moved the phone this morning.
Are you saying iOS 8 would have downloaded, but not updated until I answered another prompt? (now I can't remember what I did with 7 last night!)