Very interesting move, not only switching the media briefing to Sunday, but also announcing it so much earlier then they had in previous years.
I think the move to Sunday is a great choice. It will be much easier for many to watch. It also gives some additional breathing room between the other conferences, especially Sony.
In the past few years, one of the issues I've had with Xbox's E3 briefings has been reused trailers and demos from other conferences. Not only has Bethesda had Sunday conferences the past 2 years, but
EA was on a Sunday last year as well. This meant seeing the same Fallout 4 and BF1 trailers at MS's show 12 hours after they were show at Bethesda/EA's showing.
My one disappointment with this news, and more so the timing of the announcement, is that it more than likely points to Xbox not having a dedicated Scorpio event. This means a longer wait time to get official info on Scorpio, and also less time for showing off games at E3 (unless they opt for a longer show than their regular 90 minutes).