Well, I did this on two doors yesterday - driver's door, and rear passenger side door.
The driver's door was a total nightmare!
As excellent as the instructions in the first post are, they omit some fairly critical details about how and where to detach the rods linking the actuator assembly to the external door handle/lock. This was very hard - I could barely get my hands up there, and visibility is pretty much nil. And getting it back on again was even harder.
In case it helps someone in future, there are two metal rods going to the assembly. They each clip into a plastic hole on the inside of the exterior handle/lock assembly. The heavier rod slightly back (on the car) compared to the lighter one. Each rod works via a right angle at the end of the rod which inserts into a hole, and then a plastic clip swings over the vertical part of the rod and clips in place to hold it there. So to get each one off, you have to swing the plastic clip around so that it's no longer clipped over the vertical part of the rod, then pull the rod out (towards the centre of the car) and carefully bring it down. To get it back in (much harder) you have to do the reverse.
This isn't an issue for doors without an external key lock.
The other issue with the driver's door, and I assume the front passenger door, is that in my car at least there is a diagonal strut inside the door which makes it virtually impossible to get the lock assembly out of the door cavity. In the end I had to unbolt the lower end of this strut and then (with considerable effort) push it in and away from the lock assembly. Even having done that it was extremely hard to get the assembly out of the door.
As others have experienced, the screws holding the lock into the door were ridiculously tight on my car, too. I nearly stripped one of them, and all of them required serious abuse to get them to move. You need a bigger than usual phillips head - bigger than a #2, ideally.
In the end getting the damn lock out of the driver's door and then getting it back in again took me the best part of two hours!
In contrast, cracking open the assembly, swapping out the motors etc was a piece of cake. Mine had no crud inside the white casing, but there was evidence of soot/black crap inside the motors. The motors I ordered from ebay were a perfect fit and worked first time. My guess is that the Subaru motors are just cheap and crappy and burn out naturally over time.
I used the bolt through the centre of the actuator assembly to re-seal it, as suggested above - worked pefectly (so far).
The rear passenger side door took me a total of about 20 minutes. No need to disconnect from an external keylock,and a fairly simple cable pull system to the door handle instead. No god damned diagonal strut getting in the way either!
When I'd finished I could get the doors to unlock, but they wouldn't lock. I'd thought I'd done something really bad, but after a few minutes of panic I realised that the car simply doesn't lock if one of the doors in open!
In the end, a frustrating morning thanks to the driver's door being such a bastard to work with, but very happy to repair two annoying locks for a total price of $24 rather than $500ish via Subaru! Now have four door locks that clunk satisfyingly open and closed with the remote.