We found our first bug!
If you upgrade to premium, there should be an Edit and + button at the top of the signature list to allow you to edit/delete a signature or add a new one. These buttons are missing after the upgrade.
To restore the buttons, close the app and restart it and the buttons should reappear.
Closing and restarting the app means tapping the Home button once to get to the Home screen. Then tap the Home button twice quickly and there should be a list of apps at the bottom of the screen. Next, tap and hold the signature app icon until you see a red and white circle at the top left corner of the app. Tap the red and white circle and it will close the app the app. Tap the Home button to return to the Home screen and relaunch the Signature app.
Needless to say, bug fix is on the way!