Interesting topic...
If Barnabas had been recasted.....oh, the horror. He's not recast-able. It would never have worked. And Julia, Quentin, and Angelique were so popular that I don't see any recast working, because the audience wouldn't accept it (as in the case with Barnabas). The Victoria recast was pretty much a disaster...but sometimes I wonder if they had casted, say, Kate Jackson in the role of Victoria, if it would've worked and the character would've been kept on the show. Joan Bennett is Elizabeth for me, but in the later years of the show, I wonder if recasting her character would've been as big of a deal as it would've been in the first year or so.
My personal favorite actresses are Nancy Barrett and Kathryn Leigh Scott, so it's hard for me to imagine them being recasted. It would be so hard for me to watch anyone but John Karlen as Willie.
I just can't imagine most of the characters being recasted...I like too many of the actors and actresses.