"Did Keith Mitchell change his name, or did the BBC get it wrong?"

I know it was a long time ago and there are probably not many people still around who saw both the 1970 mini-series and the 1972 film.
I did see both, and I remember at the time being puzzled by the fact that the actor was billed as Keith Mitchell in the BBC mini-series but as Keith Michell in the film.
Hence the question, "Did Keith Mitchell change his name, or did the BBC get it wrong?"