[iOS]If I change devices, will the Leaf Tickets or other items I've purchased disappear?

If you linked Animal Crossing: Pocket Camp to your Nintendo Account on your old device, you can continue to use your Leaf Tickets and other purchased items if you link your new device to your Nintendo Account.
 
However, please be aware that Leaf Tickets obtained or purchased in the game on operating systems other than iOS are not compatible with and cannot be used on iOS devices.
 
Please see the answer to the Can I still use my Animal Crossing: Pocket Camp save data if I get a new device? question in this FAQ for more information on the procedure for linking a Nintendo Account.