I did change the Inner and Outer Fairing on my 1989 Ultra to the 1996 type fairing.
The fairings will bolt up to the triple tree but...
The major changes the Gages (Speedometer/Tachometer), Ignition switch, and in the top of the triple tree. The gages will need to be changes to match the inner fairing.
If you don't change the triple tree, the ignition switch and Fork Lock will not work. The switch need to be the newer one to match the inner fairing. When you change the assembly, you will still loose the front fork lock unless you modify the Lock Plate. I made a plate and welded it to the top of the frame a the fork pivot. This was so I could use the fork lock on the new switch.
You can use the original wiring, but I changed to the later model wiring so I could use all of the late model instruments.