I recently received an email offer for the 50,000 mile initial sign-up bonus for the Chase United Mileage Plus Explorer card. The interesting part of the offer was that it said to apply by February 29th. I knew that was only a couple days before the 3/3 integration between UA and CO's computer systems, and I knew I would be getting a new Mileage Plus number around that date, but I didn't want to miss the bonus, so I applied for the card on 2/29. I received the card in the mail today, and when I called to activate it, I asked them to verify my MP number linked to the account. It was still my old number, but since by now I have my new number, I asked them to link that number to the account instead. The guy on the phone said I was the first person he's talked to who has mentioned this (old number/new number), and I thought that was interesting. I didn't want to receive my sign-up bonus miles in 6-8 weeks, and have the old number be invalid, and have an issue with the miles posting. Has anyone else thought about this or dealt with this? I don't think there are any negatives to what I did, but the fact that he said I'm the first one is making me second guess myself!