Cannot detach/extend root volume

Issue

Our current backup/restore process has to recreate the instance since the root volume cannot be detached. This has proved to be problematic as the IPs for the instance go back to pool and have in several cases been given out to other instances. To work-around this we just have to be very careful or what’s happened a few times is the new VMs that get the IPs we need for restore get destroyed and have to be reordered – bad customer experience. If we could merely strip the volumes from the instance and then reattach - we could leave the guest in tact and have no potential overlap of IPs. How can we detach/remove a boot volume from an instance ?