Our Magic Kingdom opening time was changed from 9 a.m. to 8 a.m. Does that mean that Main Street is going to be open at 7 a.m.? If so, that's awesome. Thanks!
No, the “Main Street open an hour before park opening” thing only applies to 9AM park openings. For an 8AM opening, the front gates will open 7:45ish. “Ish” is a key word as exact procedures can change day to day. The welcome show will be on the castle stage at 7:55AM, after which they’ll “drop the ropes” at the various spokes around the hub, and the park will be open.
We did an 8AM opening when we were there the first week of April. I have a pic of us on the Omnibus at 8:09AM. I am not sure if that’s always a given at that hour, so YMMV - but hope that helps!
What time do you recommend we arrive if we would want to grab some breakfast at Starbucks before the Castle welcome show?
To do that, you'll have to be one of the first people through the train tunnel and make a beeline for Starbucks. I'd be there by 7:00 a.m. or so to ensure I was at the front. Still, you may have 10 min before the show and you may only have 5 min, so you may have to decide which is more important...breakfast or opening show.
I’ll echo the PPs, IMHO, your chances of being able to time getting Starbucks breakfast AND see the welcome show are going to be very hard to do on an 8AM opening morning. You only have around 10 minutes to play with there (if that). Maybe if you’re one of the first five people in the door at Starbucks, you could make it. But almost zero room for error. At that hour, we make sure to coffee-up elsewhere.