You can rekey a lock quickly and simply, in an evening — it might qualify as a “One-Beer” project, if you’ve got just one lock to rekey. Change-A-Lock and other companies make kits that allow you to change the key for a lock. But if you want a key you already have to fit all your doors, you’ll probably have to either hire a locksmith or do the work yourself.
My dad always rekeyed our locks to all fit the same key. He would match up the brand and model of the locks in the doors to make things easy, but if he was buying any locks, he followed a simple rule: “Buy the one with the tallest key.” With a tall key you may only have to file down the pins to get the other locks to fit the key — and taller key means shorter pins.
You can always mix and match the pins if it makes the job easier, but take pictures of the locks before you take them completely apart, just in case you don’t remember how they looked before you started.
If you have a short key, and no choice about it, or if you ran out of long pins and need one more, or if you lose a pin, then you need something to occupy the open space unless you’re ok with a less secure lock. The right diameter solid copper wire makes a good replacement, and solder wire should be ok. Make sure you’re using something that isn’t going to corrode and isn’t too soft.
File the pins to the right length, and test repeatedly before you put the lock back in the door. If you have to, you can buy extra pins for the locks, but getting the right pins without paying a ridiculous price can be difficult.
Rekeying Basics [How Stuff Works]