While most actors remained constant on the series, Psych recast young Shawn’s actor in season 5. In USA Network’s detective-comedy series, the child version of James Roday’s Shawn Spencer was played by two different actors in the first two episodes before the show ultimately settled on Liam James as its young Shawn early in season 1. Later, Skyler Gisondo suddenly took over the role in Psych season 5. Gisondo continued to play young Shawn until the season 6 finale.

https://screenrant.com/psych-young-shawn-recast-reason-liam-james/