I feel like I didn't understand it the first time around. How exactly did the retcon happen and how far back did it overwrite???

The Treasure/House/Plot Hole Juju gives John the ability to become unstuck from Canon. This is very likely either an hypercharge of his Breath Powers (Detachment, Plot), or a combination of all 4 Beta Kid powers into the Juju (Breath, Light (Alpha, Fate), Space and Time), letting him go back, not in time, but in the story to rewrite events that have happened.

Ultimately though, the only thing he ends up rewriting is the Game Over Timeline, and Vriska’s death.