I was looking at the same upgrade. Called Edge to price out the parts needed and was disappointed to learn I would have to buy a new EGT probe to match the new interface. I wanted to do a fuel pressure sensor as well. All the new parts to install the Edge CTS2 made it cost prohibitive to me for a modest upgrade. If you give them a call at 888-360-3343, they'll will give you a complete list of parts needed and pricing. They had a trade in program a while back. You could also sell your current setup to cover part of the cost. I still might get one for the larger screen and faster boot up time.
Good news! I don't know why they told you that, but you were misinformed by whomever you spoke to at Edge. I purchased an Edge CTS2 #84130, and just switched it out with my Edge CTS. I was very happy to find that it was basically simple plug and play! The CTS as you know, has two cables that plug in the back. The cable with the large plug goes to the OBD2 port, and the cable with the small plug goes through the firewall into my engine compartment, where I have two Edge add-ons: an EGT probe and a fuel pressure device connected to the fuel input on my CP3.
I removed the CTS monitor and the cable that ran down to the OBD2 port. I ran the new cable, which has an HDMI plug that plugs into the back of the CTS2 with the OBD2 plug on the other end. There is also a small female plug integrated into this cable to the OBD2 plug, and the small plug that used to plug into the back of the CTS, plugs right into that cable. There is also a female RCA plug for a backup camera on the CTS2, identical to the one on the CTS. The CTS2 has the identical slots on the back for mounting as the CTS.
I routed the new cable and plugged the existing small plug on the cable from my engine compartment, into the receptacle for it built into the main CTS2 cable, then plugged that into the OBD2 port, plugged in my backup camera, and mounted the CTS2 on my existing mount. I turned the ignition switch on and followed the prompts and the CTS2 is reading every single item that the CTS was reading, including my add-ons of EGT and fuel pressure. And my backup camera (aftermarket, not the pricey Edge backup camera) works perfectly and is much more useful with the larger screen.
I bought an "open box" CTS2 #84130 on ebay for $299 shipped. It carries a 90 warranty. Even though it was an "open box" item, it came shrink wrapped in the OEM box and could not be distinguished from brand new, not a mark on it, and everything was there. This ebay seller I bought it from seems to put 10 or so of these up there from time to time. They sell out quick whenever he puts them up on ebay.
I'm going to sell my CTS monitor with cable on ebay. They have a good market there, so even after ebay fees, I should come out with probably about a $150 net cost for the upgrade to the CTS2, and it is very easily worth all of that to me!