I am assuming you are staying onsite at one of the Universal resorts. Early entry is a perk for those staying onsite. If the early entry says 8 am. Be there at 7:30. They will usually let everyone in a little early.
June is when people start vacationing with their children here in the US so parks will probably be busy. Take advantage of single rider lines if needed and this will shorten wait times on rides.
Take advantage of your time in Hogsmeade IOA to wonder the shops have a meal at Three Broomsticks and a have a butterbeer.
I would be on the Hogwarts Express as soon as it starts running to Diagon Alley. Ride Escape from Gringottts and see Ollivanders wand shop show. Look through all the shops in Diagon Alley. There are things easily missed if Diagon Alley is full of people. Like the snake speaking parseltongue in the side window of the Magical Menagerie shop.
You have to have a park to park ticket to ride Hogwarts Express.
Have a meal in Leaky Cauldren at Diagon Alley.
There is a sticky on the Universal boards of all the things to look for in the Potter Parks. Have a glance at that. Will help your experience there be more complete.
A lot of people love the interactive wands. They are about $ 50.00 with taxes and there are sites in Hogsmeade and Diagon Ally for you to do spells. Map is in the box marking spell sites.
Universal does sometimes change their early entry parks on certain days. So to be sure you can find out from your onsite resort day before. Early entry park signs are also posted at the water taxi entrances.
Have a great time. If you have anymore questions someone here will help you.
Have a great time at Universal