I am even more impressed with the fact that so few actors were changed throughout the films. Dumbledore is the biggest one, but that couldn't have been helped. The only casting change I can think of in this movie was Hermoine's parents who had a brief appearance in Chamber of Secrets.
Help me out here but wasn't the Young Tom Riddle two different actors as well? The one from Chamber of Secrets should have been nearly the same age as the one in Slughorn's memory.
EDIT: I just chedked IMBD and they were in fact different actors from the Chamber of Secrets and Half Blood Prince. I suppose there could have been an age difference between the two Tom Riddle's but I still think they'd be nearly the same "age" in both memories/incarnations. Of course since the movies were 6 years apart it may not have been practical to try and use the same actor again.
That is true, but as you said, it was quite hard to keep the same actor. I think it works fine since I believe the HBP Riddle memory occurs before he created the diary horcrux and thus the Riddle we see in Chamber would be older than the one we see in HBP. As for the overall series, the vast majority of the adult actors played the same roles throughout.