What does the phrase change goal post mean?

Perhaps you refer to the phrase "moving the goal posts".

It means quite simply the moving or changing some objective or target. It can be used in two contexts: to imply cheating (as in someone's making a game easier or harder for someone) or to note some change of plans (usually because the original objective has become difficult or unreachable for some reason). Since goal posts are supposed to be permanent fixtures in a gaming field, the phrase has a generally negative connotation.