If your going from Android to Android, just make sure that you log into the Google Marketplace with the same Google account. You can get it from the Marketplace, and it won’t ask you to pay.
The purchase of the app is tied to your Google account, and not the specific phone.
If you are moving from iPhone to iPhone, the same applies, only with your apple account, and from AppStore.
If you are switching between iPhone and Android, then unfortunately you would have to re-purchase the app. Any extra content that you buy (like public land and GMU maps) are connected to your GPSHuntFish.com account, so those will transfer from any phone to any other phone as long as you login to the application with the same GPSHuntFish.com user.
Before swapping phones, you should make sure that you have logged in or registered a GPSHuntFish.com user, and that you have synced your trips. That will ensure that they are backed up on GPSHuntFish.com, and that they will be downloaded to your new phone when you login there.