If you decide to change device (getting a new phone or tablet, perhaps) of the same operating system (iPhone to iPhone, Android tablet to another Android tablet, etc.) then you will be able to unlock any of your previous purchases as long as you are logged in to the same account.

It isn't (currently) possible to transfer purchases from one system to another, although this is something we are looking into for the future.

Your progress through any packs, and your progress on any daily games won't transfer to the new device, but the purchases of the packs (and the ad-free daily games option) will unlock for you.