That was possible in r7.x. However, that is no longer doable in erwin r8.x, because r8 now has transformations. In r8 both sides will be the same once the Rollup transform is done, and that cannot be reversed.
NOTE: the r7.x transforms functionality has been significantly changed for erwin r8. r8 now utilizes transformations, which is a "non-live" version of a transform that is applied to certain constructs of the model, but no transform objects are created and no live updates of source and target objects are maintained. For more information regarding the transformations in r8, please see the online Help topic ' Working with Transformations '.