Moreover, I had subscribed to an unlimited offer for the backup of 2 devices. I was thinking that if I replaced an old device by a new one, I could stop the backup of the old one and transfer it to the new one. But it was not possible.
Consequently, the unlimited offer for 2 devices has been transformed in an annual payment only able to backup the 2 initial devices (one has died and the other is in the end of its life).