Changing the key and cylinder by itself won't answer your problem unless you just luck out and pick the right resistor code. There are 15 possible combinations that can apply and from what I know, it's not listed anywhere inside the car. You either need to get 15 keys (one for each resistor) or jumper the various resistor combinations into the wiring and see which one works then get a set of keys with that resistor on it.
There is a list of the various resistor combinations on them on the web. To do the resistor test, there's a connector coming out of the bottom of the column. You disconnect the wiring where the connector is (about 12" from the column) and plug each combination into the end that goes up behind the DIC and try the key. Each time it fails you have to wait about 5 minutes before trying the next combination.
As far as the ignition key goes, a good locksmith should be able to make a replacement key.
Here's a website with some info that may help:
How to determine the correct vats code value when you lost all your vats keys
The following Chart shows the Vats Code values and there respective resistor values in ohms. Divide the ohm values by 1000 to convert the value to kohms. Example: 402 ohms converted to kohms is 402/1000 = .402 kohms. Or if your meter is displaying the value in kohms you can convert kohms to ohms by multiplying by 1000. Example: .402 kohms converted to ohms is .402 x 1000 = 402.
*
CODE
RESISTOR VALUE IN OHMS
1
402
*
2
523
*
3
681
**
4
887
**
5
1130
**
6
1470
**
7
1870
**
8
2370
**
9
3010
**
10
3740
**
11
4750
**
12
6040
**
13
7500
**
14
9530
**
15
11800