Most of the carrying cases should be good, they still similar in size. The power cord is now a micro usb slot so you wouldn't need the unique one it was using before. Changing to the micro usb makes the official cradles no longer work. Grips no longer work due to the smaller size.
I think existing screen protectors would work, but new ones would fit the new glass panel better.
That's for the stuff that is not compatible lol.
I hope they create new cradles since i love leaving my Vita on one to charge. I would like to see Sony make an official grip this time, most of the 1000s grips look kinda cheap.