Delta no speaka da truth. United's Premiers and above can use the 500 mile certs to upgrade themselves and one traveling companion on ANY published fare. And if using miles from your account at the gate, United now permits domestic upgrades on any fare as well.
And Rudi, a slight correction on your prices for purchasing 500 mile upgrade certs: $200 is the price for non-Premiers. The price for Premiers is $125 for a book of four.