Worf's forehead ridges were completely changed after the first couple of seasons. Also his sash was changed.
And Kurn's forehead was always changed to match it.
Captain Kathryn wrote:
It seems as though, on TNG, she was stifling her real accent or something. At least in the first episode anyway.
According to Sirtis, she was told she had to come up with a slightly alien accent because Stewart had an English accent and they didn't want two Brits on the bridge, or something like that (despite Picard being French). So she worked out a Betazoid accent which, only a few episodes into the series, is completely contradicted by Majel Barrett.