You CANNOT upgrade upon arrival. And the 48 hrs people have been quoting is for DVC. All "regular" folks have to add/upgrade/downgrade dining 72 hrs before check-in.
Seasonal pricing no longer exists. There are one or two places that have an upcharge for Thanksgiving and Christmas meals, but they will be listed on rustyscupper's list. You can trust the prices on the sticky as it is a CM who is keeping them up-to-date.
You have to go through the special offers page from the beginning of your reservation. From WDW's home page, scroll down to the bottom and click on special offers. Once FD is announced, there will be a link to follow and then all of it will load for you and add in automatically. If you don't go through special offers, you do NOT have
free dining.