This is correct. Your GTM's will not sync thier changes unless they are running the same version. In 10.x, I have even seen the behavior that they will not even sync if the minor version is not the same. i.e. 10.0.0 and 10.0.1. Any changes made on your GTM's will have to be performed on th 10.x GTM to make sure the changes are moved over once you upgrade the 9.x GTM. You also may want to look at SOL10449 as they suggest shutting down gtmd and big3d prior to performing the upgrade.