[iOS]If I change devices, will the Orbs I've purchased or Heroes I've summoned disappear?

You can continue to use your Orbs, summoned Heroes, obtained items if you link your new device to your Nintendo Account.

However, please be aware that Orbs purchased/obtained on operating systems other than iOS are not compatible with and cannot be used on iOS devices.

Please see the answer to the Is it possible to continue using Fire Emblem Heroes as the same user if I change devices or obtain a new one? question for more information on the procedure for linking a Nintendo Account.