Rather disappointingly, after getting Barclay's World Elite MCard partially for the purpose of upgrading from plain Emerald Club to Executive, and finding the page on National's site that details the program (www.nationalcar.com/offer/mcworldelite), it appears that was only a temporary promotion. The link for upgrading on this page (https://www.emeraldaisle.com/mcelite/executive) goes to a page that confuses the issue, so I called Emerald Club and after 30 minutes of getting the agent to find the page, bring her manager in, etc., was told it wasn't applicable anymore. Just a bit frustrating, especially given that many travel websites mention this (small) perk in conjunction with World Elite cards.

If anyone else has more informative thoughts on this, please shout it out!