I have a backup for Boot Volume. I can create new Boot Volume from it and attach to a new instance.
But I need to save the existing one with all settings. Is it possible to just replace one Boot Volume to another?
Yes, recently they have added the support to replace the boot volume and they have rollback for this in place. Refer to this: https://docs.oracle.com/en-us/iaas/Content/Compute/Tasks/replacingbootvolume.htm