You just have to follow these instructions:
- Connect your new device with the same Google Play account used to buy the full version (or your Amazon account if you have purchased on the Amazon Store)
- Download the app on the Google play (or Amazon Store)
- Launch the app
- Click on the button "Unlock the full version", then "Restore the full version "
(Note that even if you do the opration of buying the full version, it will not be charged another time as long as you use the same Google Play account (or Amazon))