if that account is reported "closed by credit grantor" on your credit report, your FICO score will drop for 2 reasons:
1. The average age of your active accounts will become "younger".
2. The amount of total available credit will decrease, thereby hurting the credit utilization ratio.
Since you were given a higher limit on the new card, the credit utilization ratio will be fine. However, if you do not have many other older accounts, you may want to call Amex and get them to reactivate the old card, even with a low limit. This will keep your credit history "older" and preserve a higher FICO score. Even if they report the new account as being open since your "member since" date, you still want the old account active.