I ran into a variation of this problem last year, when I'd bought e-tickets well in advance and then had to have my credit card cancelled and reissued because of some attempted fraudulent use. I cut up the old card and didn't have any problems until I was ready to travel -- then I realized I didn't have the old card to show at the airport.

I called UA, and they told me to bring a copy of the credit card statement showing the purchase; I did, and that did the trick. I'm just glad I don't have my credit card statement e-mailed to me -- I don't know if they'd've accepted something printed on plain white paper as being official enough.