A number of readers have commented in response to my original post back in June that the steps I described there for moving credit lines between accounts no longer worked; the option to "Transfer Available Credit to Another Card," present back in June, has since disappeared from the "Manage Credit Limit" console.
Rather than continue the discussion there, I thought it was worth writing a quick update to point out that Flyertalker yugi recently shared a working link directly to American Express's credit reallocation page.
To access the page, you'll need to first log into your American Express account, then open this working link.
It's unclear to me whether the page was left up intentionally or not, so it may not be available for long; if you need to reallocate your available credit, I'd take advantage of the opportunity as quickly as possible.
As a reminder, all the restrictions I wrote about in this post still apply: the card "contributing" credit has to be open for at least 12 months; credit lines can only be reallocated between consumer cards and from consumer cards to business cards (not vice versa); and you can only move "available" credit (so pay off your card before attempting to move a credit line from it).