In either case, once you've installed iOS 8 you'll restore from the backup you just made.
Leave a way back
This is something we'll look at in greater detail when iOS 8 ships as well, but forewarned is forearmed. Be sure to have a copy of the latest version of iOS 7 compatible with your device. There are invariably cases where people wish they hadn't upgraded--perhaps because an app they rely on hasn't yet been updated for iOS 8 or, as it turns out, they hate change (or their older device runs more slowly with the new OS). If you act quickly, you can put things back the way they were.
What I mean by quickly is this: AppleApple "signs" versions of iOS. This tells the device that the version you wish to use is okay to use with that device. While that version is signed, you can install it on your iOS device--even if it's a version of the operating system that's earlier than the one you just installed. Alles zu Apple auf CIO.de
However--and this is very important--Apple traditionally stops signing old versions of iOS just a day or two after releasing major updates (such as iOS 8). If you were to update to iOS 8 say, a week after it ships, you might find that you can't go back to iOS 7. So, if you intend to be among the earliest adopters, it's not a bad idea to have a copy of the iOS 7.1.2 .ipsw file at hand.