I had the same questions last year when I wanted to upgrade my console sounder. The Matrix will snap right into the Pro 128 mount or you can use the new one that comes with the Matirx. You will have to change the transducer and the holes are not the same for the Matrix bracket on the transom. I simply mounted the new ducer beside where the old one was and plugged the holes with the old screws. I got the Matirx 25 and the GPS & weather sense. A MUCH improvement over the 128! The newer models have 640 vertical pixels which I understand is very crisp. MY 25 is 240x240 same as my Garmin on the bow. To see the Matrix 25 on my PT185, go to my photo library to see it mounted up. The cover you see is from Cabelas...
Let me know if have more questions or photos and I will try to help you out.
LT