For a recent challenge I did not have to mail in the physical card.
Usually I understand a fax with a legible copy of the card suffices.
Only one earlier post refers to an actual mailing since the card
could not be phtocopied. Even if you would have to mail it in
there is no cause to worry since the new cards are issued
again in Feb/Mar. So you would have a replacement pretty quick.
Due to some sort of urgency I was able to do it via E-mail, but I
think I was a special case. In any case contact UA and they
will give you instructions how to proceed. Good luck.