Litesonic, changing the chip is easy - nothing to worry about
:
Chip Change Guide
The stock chip requires a 2.5bar FPR. The Welt uses a 3bar. But for E-test, you need to pull fuel (and timing, most likely). As you pull fuel, your NOx will go up, requiring timing to be pulled.
If you have a working cat, and are that rich, then something is wrong. The Welt chips are not overly rich. It sounds like the car might not be going to closed-loop - check your coolant temp sensor.
Also, have you verified that you have a Welt chip? If you are using a 3bar FPR with stock chips, then it
will be overly rich...
-Rogue