Hope the CEL doesn't come back. I've never heard of "reflashing" the computer. You can clear codes by disconnecting the positive battery terminal for 20 minutes.
This thread deals with P0420 you can also use the search function and find more
If the "reflash" worked then great, but if it comes back
1: Go back to the dealership and get your $$ back
2: Spend some $$ to get it fixed
Good luck!
EDIT
I did a search on reflash, there are a few, but that's the first time I've heard of it, and I spend WAY too much time on CEG
