A little while back I got an intake off ebay, with the description "93-01 IMPREZA BAJA LEGACY FORESTER AIR INTAKE W/FILTER". Turns out it was a 97-01 intake, the intake manifold and/or throttle body changed somewhat in there and things have moved. This made the job considerably more "fun" than it should have been...
The single largest problem in the install is that in between these model years someplace, Subaru went from using Mass Air Flow (MAF) sensors to Mass Air Pressure (MAP) sensors. For the purposes of this task, the major difference is that while the MAP sensor connects (typically) to the throttle body, and has nothing to do with the intake, the MAF sensor is installed between the throttle body and the air filter.
For MAP vehicles then, the filter can just go right on the end of the intake, and everything is fine. For MAF, an adapter plate is needed. The MAF connects to the pipe, and the adapter is bolted onto the MAF, connecting to the filter. I'll have pictures of what all this looks like in the not-too-far future, I'd have them today but my camera wasn't in my bag this morning like I thought it was.
The upshot is that I ended up having to go purchase parts. The seller (toysauto) says they will refund me for half of the additional parts, which included the 3" MAF adapter, a 3" ID silicone sleeve with matching hose clamps, and a 3/8-3/8" OD hose barb nipple (male to male.) The total bill was around thirty bucks.
But wait, that's not all! Because of the design of the MAF adapter I found, there was no room for the air filter. If the car didn't have ABS, it might have fit down there, with the intake mounted sort of at an angle. My Impreza's an LS, so it does have ABS, so I cut three inches off of the intake pipe. I think two and a half would have done it, but I didn't want to cut twice as I was using a hacksaw. I made a pretty curvy cut, but it's not really a problem since that's covered up by the vinyl sleeve. Maybe someday I'll smooth it out, if I'm around a nice grinding wheel or similar.
The really interesting thing about this project is that the car is still quite quiet at low-load and low-RPM, but when you stick your foot in it, it gets louder quickly, especially when you stomp on it and it drops down into second up a hill.
Anyway, it is clearly possible to put a later-model intake into an earlier car.