No, you've got it right. It's not an unprecedented move. Ford and GM have done this before.
The new F150 represents a bunch of new stamping technologies (as did the 1997 F150) and those tools are HUGE (Google 4000 ton Schuler Press) and take a long time to incorporate into the line. So it makes sense to retool one plant while the other stays online.
GM often builds "Classic" versions (1999, 2007) marketed along side the new bodystyle, just to confuse everyone trying to order parts later on. Ford avoids this by launching their new trucks as the next model year, often coming out in January so they get the full effect. As in, January 2014 is the earliest they can call it a 2015 model (hint, hint).
