I did some research on attaching a restored boot volume to an existing instance but per this document, it might not be possible.
Here is the part I think it may not be possible.
You can use a boot volume backup to create an instance or you can attach it to another instance as a data volume.
This kind of tells me you can only create a new instance from the backup or attach it as a data volume.
Well, I’m not giving up my hope. How about cloning? I wonder if you can attach the cloned boot volume to an existing instance? I will find out.