The entirety of any high-speed signal trace must maintain the same GND reference plane from origination to termination. If the same GND reference plane is not maintained, via-stitch the GND planes together to establish continuous grounding and uniform impedance. Place these stitching vias symmetrically within 200-mils (center-to-center, closer is better) of the signal transition vias. See Figure 6-1 for an example of GND stitching via.

Figure 6-1 Ground Stitching Via Example