DR 1.2 - Upgrading old VMs for performance boost.

In the DR documentation, it mentions the following:

"Note: These optimizations do not apply to virtual machines created with VMware products prior to vSphere 4.0. For example, change tokens are not used with virtual machines created with Virtual Infrastructure 3.5 or earlier. As a result, virtual machines created with earlier VMware versions take longer to back up. "

Now, does this mean I *MUST* recreate all of my virtual machines I created under ESX 3.5 OR can I simply upgrade all of them to VM7 from VM4 to get the performance benefit from the change tokens?

Mike