Agatha Christie's Marple (or simply Marple) is a British ITV television series loosely based on the books and short stories by British crime novelist Agatha Christie. The title character was played by Geraldine McEwan from the first to third series, until her retirement from the role, and by Julia McKenzie from the fourth series onwards.
They made changes to the show as it was airing.  
The change of Marple from Geraldine McEwan to Julia McKenzie is a definite change in the show. Therefore changes were made to the show as it was airing. 
( z1 change :ARG0 ( z2 they ) :ARG1 ( z3 show ) :time ( z4 air :ARG1 z3 ) )