Written by: Vijay Kumar, Microsoft Senior Product Manager.

The 64-bit version of the Microsoft Directory Synchronization Tool (DirSync) is now available. The functionality of the 64-bit version of the directory synchronization tool is identical to the 32-bit version. The 64-bit DirSync tool uses Forefront Identity Manager (FIM) 2010 as the underlying synchronization engine. 
 

Even when the 32-bit instance of DirSync is removed, the overall state of the objects in your on-premises and cloud directories is preserved. When you install and configure your 64-bit instance of DirSync, it will find and match objects in the cloud with on-premises objects.

However, the 64-bit instance will not find and match objects in the cloud if on-premises object deletions occurred when the 32-bit instance of the directory synchronization tool was offline. Therefore, it is important to minimize changes to on-premises objects during the upgrade.

Please note that the 64-bit DirSync tool will not provide full multi-forest AD support. We will be releasing a DirSync tool that will support Multi-forest AD in the future; more details will be published closer to that time.

For instructions on how to install both the 32-bit and 64-bit versions of DirSync, please visit: http://community.office365.com/en-us/w/sso/555.aspx.