if you purchased an upgrade, you should be able to install acrobat xi on the same computer with the older acrobat (without needing the older serial number), as long as the older acrobat is still installed.
if you purchased an upgrade, and are installing on a computer that does not have the older version, you will need the older serial number or you will need to exchange your upgrade for a full product purchase.
if you need to exchange, contact adobe support: (http://www.adobe.com/support/chat/ivrchat.html).