I was able to change all the other sectors, but not the first flight
I think they may allow it if you are just a day or two off from the sale end date - depends on whether the fares are still available.
I also purchased an F ticket @ 53% discount and spoke with the contact center on the 15th (sale officially ended on the 14th) to allow me to move the entire itinerary 1 week earlier. They did it without any penalties or fare difference.
I looked at the prices and they hadn't changed - apparently while the sale officially closed on the 14th, they still had the sale prices going until the next few days.
So, my guess is if you call them while the sale prices are still available though "officially" the sale is supposed to have ended, they may still allow you to change the first leg without any penalties.