The existing speakers are 130mm. It appears the rear rainbows are 120mm Speakers. The rear Rainbows come with fitment rings also.
I guess the Rainbows are still an upgrade even if slightly smaller than the old ones.
EDIT- I had a new boxed pair of 150w RMS 4 way speakers. They are a perfect fit into the MB rings for the rear speakers if I trim the small screw fixing tabs off of the speakers. So that's what I am going to do! They are Boss Audio - and do actually sound great I've tested them before.