Board Thread:Spoilers!/@comment-2153169-20180221005515/@comment-1916997-20180221165724

Wicked Wizard of the West wrote: I think we could wait for the 7.20 title before jumping to conclusion. Personally I’m leaning towards a vignette, because it would be a waste of time for even Guinevere or Cinderella actresses to fly all the way just for attending a party or a special. A vignette would feel like a more appropriate reason for all of them guest stars to appear, they may hold the farewell party while the cast is staying here.

Perhaps the heroes defeated Gothel in episode 19, before returning to Storybrooke for the last adventure. On the plane, Henry tells Lucy the stories of other characters, with commentary of Regina. That's the reasoning this is likely not episode related though. They didn't fly the actors to Vancouver (where filming actually takes place), they were at the Once offices in California, where many of these people probably already live.