For what it's worth, I applied for the card before the 50K promo was announced. After it's announcement, I SMed Chase, asking for an upgrade to the current offer. I got a non-automated SM back saying that they received my request but he had to forward it to a different department and I should hear back within one business day.
That day came and went, and half-way through day two of waiting I decided to call the number on the back of my card because I had a large transaction I was about to make, and getting the promo meant I'd put it on my MileagePlus card instead of a different one. I explained the situation to the representative on the phone (including having previously sent a SM online) and asked if the phone rep could help. He looked up in the computer and said that I was eligible (he also commented that they typically upgrade to the best offer currently available for anyone who requests within 90 days of the card being opened...I think he was reading internal notes/guidelines and probably shouldn't have said that to me).
He had me fax in a screen shot of the offer and said it'd be processed. I faxed it in, and called a couple hours and was told that it was processed OK and my account represented the new offer.
Later that evening, I got a reply SM from Chase saying "I am pleased to inform you that because Chase wants the United MileagePlus Business Card to continue to be the Card of Choice, I issued 20,000 miles to the account. The adjustment will be reflected on your next billing statement."
I got my first statement today, and there is a bonus 20K miles on there, but they're not listed under any of the reasons (1 mile per $1 earned on all purchases, +miles earned on United purchases, +miles earned at restaurants, gas, and drug stores). They just show up as an extra 20K in the "Total Miles Transferred to United".
Does this mean I somehow got lucky and will be getting 70K miles with this signup? When I got my MileagePlus personal card, my signup miles were listed as "First Time Use Bonus"...these 20K aren't. The statement doesn't even show the 30K signup bonus I originally was presented with when I signed up, which leads me to believe that it'll be coming on my next statement (since I didn't hit the $1K/$2K minimum spend (depending on which offer I'll be getting) until two days before the account closed, I wasn't expecting it on this statement).
Anyways, moral of this (long) post: I was able to get upgraded to the new offer via phone and SM, and that the phone rep said that they typically do this up to 90 days after account opening.