Glad that Sunset is confirmed, I was afraid that, since the first movie was basically a parallel of the FiM series opener, they would have taken the Season 1 route and completely ignored her existence post-conversion.
Now, as for when the movie will be released, I'll venture a guess: the DVD will be released before or on the same day as the book (April 8), but the movie will air on The Hub only after Season 4 has ended.
The reasons I think that are simple:
1. spoiling the whole movie by releasing a book about it months before would be an unprecedented (and dumb) move on Hasbro's part.
2. there was a fully animated sequence playing at the Toy Fair, which means production is pretty much at the final stage, if not over already; in fact, only four months before the Season 4 premiere, they couldn't even show a single animated scene from the first episode, which means the time between animation and release is pretty brief.
That would make approximately a 8 months interval between the two movies.
Considering they probably didn't greenlight Equestria Girls 2 immediately after the first one, but waited a while to gauge its success, here's hoping they manage to pull off a new movie every 6 months from now on.
Edit: I believe these designs weren't posted here yet, they might as well appear in the movie:
Yet-another-Edit: Here are a few tweets that may have been related to this movie, in hindsight (courtesy of EquestriaGuy):