I would be curious how long they do this.  The only reason I am keeping my pass past my 15 months is for Epic.  So if after preview I can't get Epic, then I would drop the pass if the waiting period is more than a month or two.  Wonder how many would be like me and if they see a dip in AP.  And if I drop I plan to pick up Disney and won't be back for a couple years as I rotate thru the parks.  I know more and more people with kids my age that are now rotating thru passes instead of holding more than one at a time. 
I would be OK for a couple months, but they need to offer an upgrade to your current AP quickly when school is back in session.
Also, I hope if they do this, we get more than one day of preview.  That is is a lot during the soft openings.