From OCLC News:
The OCLC Control Number is anticipated to reach 1 billion sometime after July 1, 2013. At that point, OCLC will increase the length of the OCLC Control Number to accommodate a variable-length number string. If you use and/or store OCLC MARC bibliographic records and the OCLC Control Number, you will notice a change when the OCLC Control Number 1 billion is reached. You will need to check the systems at your institution that use OCLC MARC bibliographic records and the OCLC Control Number. You may need to implement changes to ensure those systems will be able to successfully handle the longer OCLC Control Number. OCLC recommends that you ensure your systems are ready effective July 1, 2013.