To me, this basically means that if a Silver/Gold/PP buys a full coach fare at the 6+ day mark, they'll jump ahead of the CP and get the F seat since full Y/B fares can upgrade immediately.
But they can do that at any time after purchase, whether 6 days, or 6 months.
More importantly, who buys US full Y fares these days? They're usually more expensive than discounted F.