So I got thinking there must be a good reason for that.
It would be sufficient reason if they were able to reduce the packaging for themselves. Maybe the new carton means they don't have to have those big plastic crates (the ones we used to seek out back during college - they made great furniture for the truly penny-pinching). That would be sufficient reason, but of no use to me.
On the other hand, the new carton might be good for freezing milk. But I didn't know if freezing milk is "done." Turns out it is possible, best done with skim milk, and it changes the mouth feel enough that you'll never find the dairy council recommending the practice.
But I'm just a busy mother with kids who want their milk. If having a frozen gallon or so around between shopping trips lets me buy all my milk at Costco (less than $3/gallon), that would beat the $4/gallon we are currently paying at the local grocery.
So I did the experiment. Milk expands when frozen, but the Costco cartons are thick-walled and have bunches of "grooves" in the side which allow for expansion. Plus the more columnar form of the carton means the milk can expand upwards without being forced into a bottle neck. In all, it worked pretty well.
The milk took a while to thaw. The milk ice is much more ice than milk. We started drinking milk before the ice had thawed, and that milk tasted like powdered milk. But when we let the entire gallon thaw, it tasted normal enough.
So - freezing milk is a bit of a hassle. But for our family, it is sufficiently acceptable given the $ savings.