I would have to agree with DisFreakBrent. I highly doubt that the people who bought the Harry Potter packages will be the only ones in the park from May 28 - June 17. It is just not a good idea for Universal. To close the park for that long to all regular guests would be bad for business. All that money not coming in after being in such a financial slump... I don't see that happening. Yes, I believe that the people you talk to when you call Universal will say this. After all, they are employees. They will say what they think they are expected to say when they don't have all the info, and I doubt that they have all the info.
When I called Universal yesterday I asked them several times, several different ways to confirm that package holders were exclusive to the park. They will be issued an E-Doc requiring a picture ID for the entire day, not just the 1 hour early admission. I asked over and over about others getting into the park and they said there would be restricted access points requiring these E-docs. I know they have limited info, but at this point if they change their tune anytime before 5/28 there will be a lot of annoyed paying guests on either side.