This only means that if you want your app in the App store after Feb 1 (new uploads, guess existing apps are safe for some time), you'll HAVE to build it against the 7 SDK (and XCode 5). That does not mean that you can't support previous iOS versions, you still can (google deployment target, more that that check this answer).
For now, if you buid your app against 6 SDK and run it under iOS 7 it will work in compatibility mode, which saves lives sometimes. Since Feb 1 you won't have this opportunity.
To make sure you're fine after Feb 1:
- Use XCode 5.
- Select your project file and go to Build Settings.
- Base SDK should be 7 ("Latest iOS" for now).