With the release of Azure AD Connect for synchronizing on-premises Active Directory to Azure Active Directory, existing deployments of Azure AD Sync can consider performing an in-place upgrade of their AAD Sync server to AAD Connect.
Azure AD Connect can be downloaded from Microsoft here. When you have downloaded the installation file to your directory synchronization server launch it to begin the upgrade. Setup will detect the existing install of AAD Sync. Agree to the license terms and click Continue.
Synchronization will be stopped during the upgrade. This isn't harmful to existing objects that have been synchronized, but you should be aware that no changes or new objects will synchronize during the upgrade.
Enter your admin credentials that will be used to connect to your Azure Active Directory instance.
Choose whether or not to start synchronization immediately after configuration is completed. If you have post-upgrade configuration changes to make you may prefer to untick this option.
When configuration is complete you'll see a useful tip for syncing Windows 10 domain joined computers to Azure AD as registered devices. This is an interesting feature but not directly relevant to an Exchange Online scenario so I won't go into it here.
After the upgrade is complete you'll see an Azure AD Connect icon on the server's desktop that opens a dialog from which you can perform a few admin tasks.
The applications menu/list on the server also contains other utilities such as the Synchronization Rules Editor, Synchronization Service, and the Azure AD PowerShell module, which you may already be familiar with if you've been administering directory synchronization servers in the past.
In my experience so far the upgrade from AAD Sync to AAD Connect is trouble-free and preserves all of the filtering and customizations that I have in place.