From the terms: "This one-time bonus offer is only valid for first-time United MileagePlus Explorer cardmembers with new accounts. Previous and existing United Airlines cardmembers/accounts are not eligible for this bonus offer."

The first sentence indeed should only disqualify *Explorer* card holders, but the second one seems to apply to holders of *any* UA card. "accounts" seems to mean that you can't convert an existing account (and indeed the conversion offers I've seen in postcards and online offer "10,000 miles", but none of those are instant bonus miles, just the 10K extra after you spend 25K in a year). But "existing..cardholders" would seem to mean that even a new account wouldn't qualify.

Don't get me wrong -- I'm very glad it is working out, and I would have been annoyed enough to cancel my old card and go to a different rewards card if we loyal cardholders got shafted.