Currently SD does not support this feature of merging back the changes to existing policy.
This is not supported for below reasons:
1) SD policy can be shared with multiple devices, so it may not be correct to take change from one device and apply it to all assigned devices.
2) There could be some conflicts like someone changed ip address value and after you import this change will be pushed to all policies that refers to the address object.
We are exploring some options to support basic cases of merging changes back to policy in 17.1 release.
-Srikant