Complex sentence: The character of Arthur Carlson, Jr., returns as an adult, but Sparky Marcus did not return to reprise the role that he had played as a ten-year-old in one episode of the original series.
Split into simpler sentences: The character of Arthur Carlson, Jr., returns as an adult. However, he is not portrayed by Sparky Marcus who played a 10-year old Arthur in one episode of the original series.