Kilo: degrowing VM has problem when scheduler_hints->different_host enabled

[Roger]

I've setup a stack where several VM exists.
for example, Vm1, Vm2
I put different_host rule in the yaml file, like below.
    'node:Vm2:server':
        type: 'OS::Nova::Server'
        properties:
              ......
             scheduler_hints:
                different_host:
                    -
                        get_resource: 'node:Vm1:server'

When I degrowing Vm1 from this stack, I firstly removed above line from node:Vm2:server in yaml file.
Then run "heat stack-update -f <yaml_file> <stack>".

The issue is that Vm2 will be rebuilded. All the configurations will be lost.

If I dont' remove that line, stack-update will fail since Vm1 is removed from stack.

How to resolve this?