Could you please explain retcon?
Retcon, Retroactive Continuity. It is going back and changing or adding to a previously established fact.
In Homestuck’s terms it is different than time travel. It effects the Narrative itself rather than just a timeline and as such avoids the creation of doomed timelines. Material in the Furthest Ring (dream bubbles and horror terrors) are immune to any changes likely (speculation) because they exist slightly outside the Narrative taking place almost exclusively in Intermissions.