This subject has been talked about in numerous different threads... Since I have a '15 RT+ I too have been very interested in this subject and have done some investigation as well. From what I've gathered from other people trying this upgrade:
1. It cannot be just a software update unfortunately like you have stated above.
2. To even attempt this upgrade, at first you must replace the Media Hub in the arm rest to the 2017 (has two USBs and an aux port), plug n play easy switch.
3. You MUST buy a 2017 8.4 screen FROM THE SAME TYPE of car that you have. This means if you have an RT+ you cannot buy the screen from an HC, SRT, SP, SXT, or RT, must be from an RT+. The reason being if not from the same model, you will lose some functionality like heated seats, sport mode, etc.
4. If you do find a 2017 8.4 screen from the same type of model car, take the VIN from the screen and input in the VIN into the build sheet website (I don't have the VIN website on hand, copy paste the URL then copy paste the VIN, hit enter, gives you build sheet) and it will let you know if you indeed have found a perfect match. You could find an RT+ but the new 2017 screen came from a car that wasnt a STP and then you are screwed if you current car has the STP.
5. Lets say you find THE needle in the haystack, you are probably paying anyway from $1000 - $1500 for the screen... and that's on eBay.
In my eyes its not worth the risk to buy a maybe faulty screen and take the risk on losing functionality in my car for maybe the additional possibility for Car Play. I already have Nav in my car, which i love, so it really isn't worth it to me.
Sorry to be a dream killer
Like I said earlier all of this information i have gathered was through scouring this site, in one thread two guys were trying this install and is where the majority of this information came from.
Allot of good information here but I do not agree on the same/similar model. You don't lose things like heated seats.....Pretty sure radio learns what your car has through the modules in your car.
I think some of the bigger sellers on eBay and OEMNavigation have all this stuff figured out pretty good. When buying privately or through a junk yard there are unlock codes to worry about
My 2018 had a 7" screen and I bought a 8.4 with just Android Auto and Carplay (about $600) and it was 100% plug and play. Just had to go through a reset or two. A friend of mine did the 8.4 (Nav Onlu) with no issues as well.