SPOILER........
There is also Gerard Stiles (another good to evil character) due to possession by Judah Zachery.
I might question this one, VAM. Gerard Stiles wasn't exactly what you'd call a "good" or "nice" man even before he became possessed by Judah Zachary. Remember, he also used an alias and was involved in pirating! The "Nancy Barrett" character in that storyline charitably referred to him as a "hustler" or some such (I know that wasn't the term used, but something like that) but claimed he wasn't evil. Even that characterization of him is somewhat questionable, though, IMO.
My vote for one of the most dramatically changed (and least believably changed) characters is the servant Beth in 1897. Her switch from sympathetic, caring maid to handmaiden of the dastardly Count Petofi was hard to swallow. I suspect it had less to do with her intrinsic character and more to do with the decision to extend the 1987 storyline (too much so, IMO).
Great topic, Cassandra! I, too, am glad that
some of the characters changed for the better (such as Mrs. Johnson and Carolyn), as you mention.
And even that a good, pure, innocent character, Rachel Drummond, remained true to her character (a very Dickensian character, much criticized on this forum, but that's another story ... though I'll just mention that I think it was KLS's most successful character other than Maggie).