I‘m not saying they retconned Jason’s existence. It’s that they retconned the Ewing Oil origin story.
In the first season, Jock tells Pamela his side of the story of how the feud started: Jock and Digger struck oil, and Jock decided to register the claim in his name only, in order to prevent Digger from gambling his share away. Digger misunderstood and thought Jock was trying to steal from him and went after him with a gun. So Jock decided to keep the well in retaliation, and built Ewing Oil from this well.
In season 8, the Ewing Oil origin story changed. Now, they said that Jason was a partner in the original strike, along with Jock and Digger. Jason and Digger sold their shares of Ewing Oil to Wallace. Windham, who then sold them to Jock. That contradicts the season 1 explanation, that Jock was always the sole legal owner of the well.