Are you sure about that? My book has the bolt circle and bolt diameter changing @ 4" and greater. Even if you could do it, it's a bad idea. You want your spec break downstream of the flange to avoid confusion during fabrication.
Jay165 is right, the bolt circle and bolt diameter are the same upto 3" and it is not common practice to bolt 300# to 600#. If a specbreak is needed you do it at a weld location on your isometric.
