Here's another data point..
I applied for the Gold card in late 2010 and got my 25K points. I called Membership Rewards (1-800-297-3276) and said I got the 75K offer in the mail and asked if I could just switch to the 75K offer. Lady put me on hold, then said she couldn't find it. I told her I had a bonus offer number of 5894. She put me on hold again for what seemed like ages (more than 5 mins). Then she said that I have to fax it in. I obviously didn't have the letter, but played it cool and said "ok, give me the fax number". She said "hold on while i get you a good fax number" then came back a few mins later saying "oh you don't have to fax it in.. i just added 50K points to your account"
And I just logged in to my MR account and saw my balance went up by 50K
Thanks for the tip!!