Upfronts are when ABC announces the fall lineup for the following year. They’ll say that ONCE is renewed for S4 and give a brief little speech about what’s in store (mostly higlights that there are heroes and villains and it’s a fairy tale) but they give the cast list as well.
Why would ABC spoil that before the finale even has aired? That seems like it would give away too many important spoilers.
On another note:
A&E confirmed in the recent TV Guide Lightening Round that the Knave of Hearts won’t appear in the finale of OUAT. So whatever scene was secretly filmed probably did not include Michael Socha.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy