Thought I'd make a little guide on this. It might already be common knowledge, but I couldn't see anything after a quick search.
The stock rear speakers have an awkward 3 bolt fitting, and usually require you to buy/make adaptor rings, so hopefully this should save you a few quid if you're thinking about upgrading your rear speakers. It might also work for the fronts, but I changed them a few weeks ago and already had the adaptor rings, so didn't think about it at the time.....
First up, remove the speakers - I'm sure there's already a guide covering this so I won't go into great detail. Basically, there's two screws, a screw-clip and a load of normal trim clips holding the door card on.
You will need:
A phillips screw driver
Wire strippers & crimpers
A Dremel or similar rotary tool with a cutting disc
Replacement speakers - I used a pair of Pioneer TS-G400, they fit perfectly inside the speaker rings and don't foul anything inside the door
Once you have the speaker out, it'll look like this:
Flip it over. Use the Dremel to remove the four support arms which hold the magnet etc for the standard speaker (sorry, this pic is really dark for some reason, so I've circled the bits you need to remove). Once you've cut the support arms, push the magnet out through the front of the ring and the whole speaker should pop out.
Flip the speaker ring back over and you'll be left with this:
There is a raised ring which runs all the way around the top. I placed the speaker on top and removed material from the raised ring, which allows the speaker to fit neatly inside the main speaker ring. You can probably remove the entire raised ring if you want. With the material removed, mount the speaker to the speaker ring and secure it with the supplied screws (You may want to use a small drill bit to make a pilot hole for the screws).
This should get you to this stage:
You can now go and fit the speaker to to the car using the standard hardware. Obviously you'll need to adapt the wiring in the door. The finished article should look like this: