When you change iPhone / iPad to a new model, it is a way to restore the state you purchased before
Go to the ’Application settings’
In-app purchase item should be ‘Not yet purchased’
By tapping ‘Restore’ button,
Start searching for your previous purchase history according to itunes account（Apple processing）
If the ‘Purchased’ answer comes, the in-app purchase item will be ‘Purchased’
Note) You can only restore your iTunes account when you purchased it before