So here's a quick tutorial on how to do this, it's pretty easy, should take only 30mins or so if you're used to wrenching on your car. Maybe 45mins to 1 hour if you're not. I take my time to not make mistakes so I take a full hour :)
Smaller flat head screwdriver
Philips screw driver
17mm socket and breaker bar
Do this at your OWN risk, when modifying the steering wheel or removing air bag modules, you risk damaging the air bag, deploying it or some other crazy thing. If you don't torque the wheel nut back correctly, your wheel can come off during driving and cause a deadly accident. YOU HAVE BEEN WARNED.
Use 10mm socket and remove the negative battery terminal, then press on the brake a few times. Wait about 10-15mins then come back and press on brake again to make sure there's no residual electrical current in the systems. THIS IS CRUCIAL otherwise you risk deploying the airbag when unplugging it.
There are 3 holes behind/side of the wheel. You need to use a small flathead to push against this spring loaded metal clip thing to release the airbag. If you're having trouble understanding how it works, look at your STI/WRX wheel and play with the clip and you'll see how it works.
After the 3 clip things, the air bag will pop out, there will be 3 connections to remove, a grey ground wire and 2 SRS wires, one black and one red connector. The wires for SRS are always yellow. It should look like this after you removed the airbag.
Look your steering column if you haven't done it yet. I didn't do that because I had to rotate the wheel to access the 3 holes with better angles.
Now remove the 2 connectors that control the cruise/radio and MFD, they are near the top of the wheel, white connectors.
You are ready to remove the wheel! Get a 17mm socket and a breaker bar and remove the nut. Use a quick forceful jab to remove it. The Forester seems easier to break the nut than my STI.
Make a mark on the stud in some relation to the wheel and take a picture so you know where to align it back later.
Thread back the nut on a few turns so the wheel won't fly off when you pull it. You have to pull on it hard different directions to get the off. Once is off, slowly pull the wheel out and DO NOT LET THE CLOCKSPRING rotate too much. If you do, you have to reset the clockspring via some procedure.
The old wheel with the STI one ready for transplant :D
Flip the wheel over and remove 4 bolts with the philips and 2 connectors for the paddle shifters (if applicable) and this one brown clip.
Flip the wheel back to front and remove the connector that was with the brown clip.
Also remove the MFD controls screw shown here:
Now you can just pull on the cover to flip it back to reveal the 3 hidden screws for the 2 side controls (radio, cruise ) and the bottom silver bezel thing.
Unscrew those 3 screws then pull on the 2 steering controls to remove them, they are not that tight but don't worry too much about breaking it.
Should look like this now.
2 wheels side by side naked :D The STI/WRX one is slightly smaller. Didn't measure but I estimate about 1" in diameter or so.
Now you can transfer the wiring, controls, MFD control into the new back cover. I hope you took some pictures along the way so you know how it goes LOL. Or just check my past pictures here.
If you're anal, you can trim the back cover so it doesn't bulge out like this or trim the silver bezel. I just left it as it doesn't interfere with anything.
This is the part you should trim or just get a scissors or some cutter to cut these 2 plastic tabs off. The wheel cover was made for the STI air bag which is shorter and the STI bezel piece which is bigger. So we have to trim this crap off.
Now reverse the steps and put back the pieces to the new wheel staring with the Steering controls and the bezel, then the connectors and that brown clip and remember to also screw in everything. I almost missed the MFD screw.
Lastly put back the 2 paddle shifters.
Time to put the wheel back into the car! Align it back however you marked and make sure it's correct. Again don't move the clockspring too much.
Reconnect the 2 connectors for the cruise/radio/mfd stuff at the top.
Torque the 17mm nut to about 28-30 ft-lbs
Once that's done, reconnect the airbag connectors and ground wire, then just pop the airbag back into the wheel, it should just click in.
Completion, put back the negative battery terminal and start up the car to ON only to make sure no SRS lights light up.
Make sure when starting up the car, there's no SRS light on as well, otherwise you did something wrong.