It is possible to downgrade an app from the App Store. All you have to do is to delete the app from the store and then connect the phone to iTunes on your computer. You will then have to backup and sync your stuff on iTunes. When this is done, click on the iTunes menu under DEVICES and then select the app tab. From here, you will find the app you want to downgrade and put a check on it and then click the Apply button. This will copy the old version of the app on your phone.