I haven't made 1K yet, so I can't tell you about the difference b/t 1K and PremExec. But when I was PremExec I got many more last minute upgrades than when I was a lowly Prem. The only one I missed was to/from Hawaii when the entire front cabin was PremExec or 1K.
------------------
"If you want to win the game, write the rules."