Some notes about my experience with the upgrade of a Trend Micro OfficeScan 10.6 to 11.0.

Upgrade path

Check the version requirements before upgrading: Upgrade path to OfficeScan 11.0.

Basically, be sure to have the latest version of your current major version and service pack installed.

Upgrade considerations and pre-upgrade tasks

Upgrade considerations for OfficeSCan (OSCE) servers and clients/agents

Check the system and OS requirements, as they have changed since 10.6 and 11.0. When upgrading, it is also recommended to create a backup, in case something goes wrong during the upgrade.

Upgrading from a version prior to 10, you may have to workaround losing your defined scan methods. This is not required when performing an in-place upgrade from any version 10 to 11.0.

Upgrading the OfficeSCan server

It's always a good idea to create a backup of your server before the upgrade. In this particular case, the backup gets created right after disabling automatic upgrading of clients, as for the backup to be taken properly, you need to shut down the OfficeScan services. See the steps outlined below, and the articles mentioned therein, for more details.

Follow upgrade instructions: Upgrading OfficeScan (OSCE) server and clients.

In short, follow these steps.

1. Turn off automatic client upgrading (see above link), preventing the clients from automatically downloading and upgrading to the latest version. This allows you to check whether everything on the server is up and running, and settings have migrated succesfully. If not, you can always rollback using your backup (or snapshot if on a VM).
   Note that it takes some time for all clients to retrieve the configuration change, so wait for a sufficient amount of time before going to the next step. If you upgrade the server too soon, clients that have not retrieved the configuration change, will still perform an automatic upgrade. How long you have to wait depends on the amount of clients registered on the server.
2. Create a backup of your OfficeScan installation (outlined here). If you're running inside a VM, you could also create a snapshot of the entire server.
3. Perform the upgrade, using the main installer. After the installation, download and install any remaining patches.
4. After you verified that the upgrade was succesfully completed, and the server is up and running, turn on automatic client upgrading to perform an upgrade of all clients. Alternatively, you can also perform a phased upgrade by manually upgrading clients.

After following the steps outlined, and consulting the articles I mentioned here, the upgrade went pretty smoothly. The important bit is not to skip steps and to carefully read the instructions in Trend Micro's knowledge base.