1. A method for migrating circuit layouts comprising: building a floorplan layout for a target circuit using a subset of constraints that characterize a layout structure of an original circuit, wherein at least one of said constraints represents a relationship between more than two cells; shape-constraint-based scaling the floorplan layout by scaling parts of the floorplan layout in accordance with a plurality of different scaling ratios such that portions of the floorplan layout are concurrently scaled with the plurality of different scaling ratios; placing cells at locations defined by the floorplan layout; and shape-constraint-based legalizing the floorplan layout using all of the constraints to produce a migrated layout.