I just called because we are booked, room reservation only not a package, for June 28 - July 5 and was wondering the same thing. The CM I spoke to stated that again that those who have a package reserved (as advertised from June 29 on) will be honored before July 8 and able to access Diagon Alley as promised. She could not confirm the same for onsite regular reservations.
We may go anyway hoping for a soft opening because we were in the same situation when HP opened years ago. Back then we had onsite reservations only booked about 6 months in advance for the end of May before they began selling package reservations and they changed the opening date then. Universal allowed all onsite guests to enter HP land as well as those who booked packages. We were allowed to experience the entire Harry Potter area before it ever opened to the public. I can't tell you how awesome that was!
I am going to keep calling every few days before we leave to check to see if things change. I encourage anyone booked during the same time period to do the same. I'll let you know if I hear anything.