I have an active United Mileage Plus "Visa Signature" card opened a decade ago, and all the offers I got to "upgrade" to the Explorer card were pretty crappy (no bonus miles, no fee waiver).

I was able to get the 60K offer to appear last Wednesday by logging in and out of MP a few times and pasting the link in this thread (I think it finally came up while logged in, but only when I pasted into a new tab; it took a few tries). Took screenshots of the offer, filled out the application, and got a screen saying up to 10 days for a decision.

Called the reconsideration number from this thread the next day and the agent immediately said I'd have to take some credit from the existing card and asked how much. That was fine as it's gone up to way way more than I'd ever use, so I had her put half on each. She said it was approved, and the new card number appeared at chase.com on Monday.

The key is that I got an email confirmation that "Your new Card is on its way" and "Remember, you will receive 50,000 bonus miles after the first use of your Card." So I'm pretty sure once I receive the cards that all will be well. I debated even trying it, so thought others with an existing card would be interested in confirmation that it appears we are eligible.