Enthused by other posts on changing to key in a lock barrel I decided to attack my door lock to make it the same as the Ignition and Boot lock.
1. Remove the door handle from the car.
2. Remove the collar from the handle. If you have the key, turn it till the square locking pin is inside the handle and the collar will slide off. If you don't have the key, you may have to file the Square Locking pin down till you can slide the collar off.
3. Knock out the pin that holds the Lock Barrel in. I filed the point off a suitably sized nail.
4. This is what you should now have.
5. Look at the lock barrel and you will see that all of the lock Tabs (don't know what their real name is) are being pushed out of the barrel by the springs.
6. This is how the barrel looks with the original key in. All of the Tabs are flush with both sides of the barrel.
7. Now insert the key that you want the barrel to match. Here is my Ignition Key inserted into the Door Barrel. Note that two of the Tabs poke out one side and one on the other.
8. You now need to change the Tabs that poke out either side of the barrel with ones that don't. The best that I can work out is that there are three sizes of Tabs. Where the line is in this comparison there is a 2.5mm one on the left, then 3.5 and then 4.5mm. This is the length of the solid part to the cut-out that the key slides into.
9. Without the key in push one of the Tabs out that was not flush with the Barrel from the opposite side that the little springs are on, this will be the side that the smooth side of the key is on.
10. Replace this Tab with a different one that has a shorter or longer slot in it and refit the key. You will see if you have gone the right way with the different Tab. You might have to steal Tabs from other spare locks to get the ones that you need. Don't lose the tiny springs when you are changing the Tabs
11. This is my Barrel with the Ignition key fitted and the "spare" original Tabs that had to be replaced with the correct size ones for this key. All of the Tabs are now flush with the Barrel.
12. Refit the Barrel into the door handle and replace the locating pin.
Refit the door handle to your car and now the lock should match the other ones.
As the process is essentially the same.