[villagetinker]

IMHO, start calling the various concrete painters, tell them over the phone what you want, and the ones that can do this will respond, the ones that want to do the entire driveway, you can hang up on, and the ones that come out and then say, oh you need to do everything because we cannot match (due to fading, etc.), you can determine if it is worth the cost.
We did exactly what you want to do, but we knew the original painting, and we contacted him about the work before did the expansion. Neighbors just had a similar experience where some of the driveway paint was damaged. They ended up having to have the driveway sand blasted and repainted. They indicated the cost for the sand blasting was $2/square ft. on top of the cost to repaint.