Is a Twist a Retcon?

A breach in continuity isn't automatically a retcon. If the author always planned for that twist and wove it into the story, then it's not retroactive. It all comes down to planning, an authorial intent. The writers and producers have said that they changed their minds several times during production about who would ultimately be gossip girl. This reveal retcons the whole series, but in a bad way. Without a clear end goal, the reveal was always going to be a retcon and change future re watches.