Key reprogram?

For an 03 impala you have what is known as the Passlock system. It uses a small transmitter to send a signal out to the key and reads the code from the key. The code tells the system if it's a valid key and what type of key it is. They had a key with a black head which is the master key and a key with a gray head which was the valet key. The valet key would do different functions like not unlock the trunk or glovebox or storage and could alter other things as well depending on the options.
In this case the cylinder from a 2000 should have worked if you used a scan tool or the thirty minute relearn to program the key into the system if the key was a valid working key.
The 10 minute relearn only works using a scan tool and only if you have a key that was already in the cars system.
The plain key will not work as that will only move the door locks and ignition but it doesn't have the transponder chip in it that the system can read.
In this situation there are a dew options, by far the easiest one would be to contact an automotive locksmith or the dealer and get a replacement key cut using the vehicles VIN number. That gives you a key that will work in all existing locks and you only need to program it into the system, which can be done using two methods.
One method is to use a scan tool or programmer like the dealer or locksmith will have and program it into the system.
The next is to do it yourself using the 30 minute relearn procedure. For that the process isn't hard but it does take a bit over 30 minutes so be sure the battery is good and you have the time.
For it you take a master key (black headed, not gray) and follow these steps.

1. With a master vehicle key, turn ON the ignition, with the engine OFF.
2. Observe the security indicator. After approximately 10 minutes, the telltale will turn OFF.
3. Turn OFF the ignition, and wait 5 seconds.
4. Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes total.
The next step is critical, as soon as the light stops flashing after that third cycle you must turn the key off, then wait the 5 seconds and try to start the car. The anti-theft learns the key and enters it as a valid code at that point. If you don't turn the key off first it will not accept the programming and you have to start over. If the car starts and runs the key is now a valid key and you can use it to add other keys(including a valet key) to the system using the following process

1. With a previously learned master key, turn ON the ignition, with the engine OFF.
2. Turn OFF the ignition and remove the key.
3. Within 10 seconds insert the new key to be learned. Turn ON the ignition with the engine
OFF. Turn the key off then try to start the engine with the new key. The vehicle has now learned the new key if the engine starts and runs.

In the case of changing the ignition cylinder, if that is the only change then the thirty minute relearn will normally work. However do not use that to add new keys as the 30 minute routine also erases all keys in the system so you may have one key that works but now the other 2 you gave family members will not work without adding them back in.

This process only applies to the transponder keys and will not work on remotes or keyless entry systems, those need to be done using different methods.