Mine also just changed from preparing for shipment back to processing. I preordered a 7+ rose gold 128gb.
I recall this happening to folks during the Apple Watch launch (and it seemed to occur by lots of certain sizes / band colors). and all was fine in the end.
Just happened to me with a series 7 titanium watch….. was preparing to ship to be here Oct 18…. Now back to processing with an estimated delivery of Oct 28 - Nov 1st. Oh well, first world problems right? Lol