Since the vast majority of upgraders are Premiers as well, I suspect that most people who are upgrading to F simply use the Premier line. I certainly never have used the 1K/Full Fare First line in SEA, and I almost always upgrade. I don't really have a problem with not using it; I figure that if I spent $300 on a ticket that I upgraded to F, I can live with a bit longer line.

Once, when I was flying F, connecting to a C out of Chicago to Europe (that happens automatically if you buy a C ticket from SEA to Europe via ORD), the agent at the Premier line told me that I could have used the Full Fare First line, since I was considered to be Full Fare First. That was nice of her to point out that I had waited 2 extra minutes in line .