It does depend on the hearing aid. If you have GN Resound aids then there is a setting in the Smart 3D app which allows you to select “Optimised for MFI.”
This disconnects the app when it goes into the background and is supposed to allow you simply to swap between IOS devices, for example iPhone to iPad.
However I found that it’s quite unpredictable whether the aid will connect to phone or tablet and sometimes one aid would connect to one and the other aid connect to the other.
Also waiting for the app to reconnect every time you bring it onto the screen can take 15-20 seconds, which is a long time if you are trying to stop streaming for example, or reply to someone.
In any case if you turn the Bluetooth off, the aids will disconnect (which isn’t un-pair) from that IOS device and when you turn Bluetooth on on the other IOS device, they should connect to that. Whether you will have to restart the aids is another question. With Resound I found the One would generally reconnect ok but the Omnia often needs you to restart the aids.