Hi James, yes Brad is right in that the easiest way is to pull the zipper seam and fill the baffle openings created along this seam. Is the Phantom a full-length zip? Is there a cross-block baffle on the seam opposite the zip, or is there none, which would allow shifting down from top to bottom? If there is a full zip and no cross-block baffle it becomes much easier; the baffles can be filled from one side. If there is a cross-block baffle, the fill must be added to top and bottom. If the zip is not full-length, there will be a seam below the zip that must be opened to access the lower baffles. Overfilling the hood and footbox will be another magnitude of difficulty, and something I wouldn't try personally; you would move from one or two open seams to the bag nearly de-contructed if opening the hood and footbox dependant on their design and pattern.
Not sure of the zipper size, but if it's a five or bigger, you could replace it with a #3 while you've got the old zip removed. The weight loss of the lighter zip will offset the additional down to a certain extent.
Worked with down before? There are a few tricks to make the job easier; read Roger Caffin's MYOG article on down quilts and bags as he has detailed all the problem spots and how to deal with them.