Upgrading to the new iWork
In case the MAS isn't recognizing your copies of iWork, I wouldn't buy them yet. According to Macworld the issue is being worked on.
If you own iLife '11 or iWork ’09 apps for the Mac but did not purchase them from the Mac App Store: You should be able to download the latest versions for free by going to the Mac App Store and tapping Update, though you may need to first visit the Purchases screen, where you might see a box called "You Have [number] Apps to Accept." This ties apps you have on your computer to both your Apple ID and a hardware identifier on your computer, and it should allow you to download free updates to these programs from any computer associated with your Apple ID.
Some users (including yours truly) are reporting issues where the Mac App Store doesn’t recognize their copies; when they click on an iLife or iWork app’s price, they receive the following alert: "[App] is already installed and was not purchased from the Mac App Store. Do you want to buy [app]?" Macworld spoke with an Apple spokesperson who noted that there’s currently an issue with some legacy customers, and the company is working on a fix.