It has been stated that "Last Salute to the Commodore"
was intended to be the final episode of the series (this is the one where the "arrogant and googy" Columbo replace the old "modest and mildly eccentric" version. Somehow, it was decided to keep the show going, but far fewer episodes were made in seasons 6 and 7. Thus, I think it is clear that Peter Falk was getting tired of the role, and, although, I am no expert in the art of acting, it just may be the changes in Columbo's personality were do to Falk making less effort in the role (however, there are still some fine episodes where "modest and mildly eccentric" Columbo reappears, such as "Fade into Murder" and "The Conspirators").

Often the script contributes to the character as well. Columbo could obviously not be his usual self in the inappropriate "Undercover." However, "Rest, In Peace, Mrs. Columbo" was a classic--spirited almost as if it were a 1970s episode.

There seemed to be different techniques toward the latter portion of the original series (pulling clues out of pool table pockets in "How to Dial a Murder" and going out drinking with the villain in "The Conspirators.")

Generally, the acting has been satisfactory. It was also great to see William Shatner again in "Butterfly in Shades of Grey," an episode which was actually better than Shatner's NBC "Fade in to Murder" counterpart.