In my experience - for all intents and purposes, the park is closed when the parade starts. The parade concludes and the concert begins on the main stage. Sometimes if you are watching from the back of the park (e.g. near Beetlejuice) then you will miss the first couple of songs by the time you walk to the stage area.
Most people stake out their spots to either get a good view of the parade or a good view of the concert. It's hard to do both. My happy medium is usually watching from near Monsters Cafe through to the Macy's building - and then just walking up to the concert area and viewing it from the outer perimeter. It's all good fun.
Hope that helps! Have a great time. We saw Trace a few years ago and he was awesome. I've got a great video of him singing Honky Tonk Badonkadonk!