You can use the same save data on your new device by linking the save data with which you are currently playing and your Nintendo Account ahead of time.
■ See the following for instructions on how to link your save data.
How do I transfer my save data? (When changing to a new device, etc.)
"If I reinstall the Fire Emblem Heroes application, will the Orbs I've purchased or Heroes I've summoned disappear?"
"If I change devices, will the Orbs I've purchased or Heroes I've summoned disappear?"