This is a tough call for me...
I loved Richard Harris, but I also became used to his acting style, his voice etc. So, in my mind, he will always be THE Dumbledore.
Now, Gambon did do a good job - but it will take me awhile to get used to HIS style. It is different, which makes Dumbledore seem different. When you get used to a person playing a certain character, it is hard to switch to automatically liking a new person playing that same character. Hopefully after I see HP #4, I will be warming up to him and his style for the character more.
So, if I am having a hard time dealing with this change, think what will happen if any of the kids are replaced in future HP movies (if a 5th, 6th and 7th movie are made). I just don't think I could handle THAT big of a change!