I’m sure it’s fun for the grads but I’m not a big fan of the event as a visitor. It tends to bring some of the least considerate party guests I’ve encountered at Disneyland, unfortunately.
Oh my stars. This affects the dates I’m considering. 
It’s not worth changing anything. Or at most, I would say just try to be at Disneyland park on a Grad Nites day, at least for the evening.
Agree with @Jeff_AZ, it’s not worth changing up dates. We’ve had a mix on Grad Nites. Sometimes you get a great group of respectful brand new adults who love DL and really are there to soak in the magic and are really sweet to & notice the families/kids. And on the other end you get rowdy groups of older teens that decide the best place to congregate in a giant circle is over the entire walkway and will attempt to hurdle your stroller as you try to navigate between their complete obliviousness to your presence as a guest in the park.
And I actually find that the middle school & high school large groups (like there as band/choir/dance/etc.) that tend to come in March/April are a lot more troublesome and tend to be worse than the grad nite crowds. Plus, there is no schedule of them coming so you just kind of find out when you show up if there’s a school group or not and hope they’re well chaperoned & have a smidge of maturity to be respectful of the people in the spaces around them.
ETA: The hardest part about grad nite actually is using a women’s restroom! That’s another place they seem to congregate at the mirrors and such. Avoid the Main Street bathrooms especially, or really any smaller bathrooms after 3-4pm.