From what I can tell from the documentation, while you can upgrade the clients to a version 7 of TRIM, TRIM cannot be operational. The upgrade process (from v6 to v7) states to upgrade the clients and then the servers, but also indicates that TRIM should not be available to the users during this process.
Here's the info concerning Mixed Environments, straight out of the Install document:
Mixed environments To minimize downtime in very large organizations, HP TRIM 7.x runs in mixed environments, i.e. environments with clients and servers using the same version of HP TRIM (e.g. 7), but different build numbers. 2.0.x represents the build number. You can see version and build numbers in the HP TRIM client by clicking Help Setup Information Software Versions.
However, as a general rule, HP TRIM clients in mixed environments must have build numbers the same or later than the HP TRIM Workgroup Server/s they communicate with. For example, with a Workgroup Server on version 184.108.40.206, the clients should not use any build of version 7.0 or 6. However, they could use HP TRIM 220.127.116.11, for example. Without this ability, downtime would include the time it takes to roll out HP TRIM 7 to all clients.
With this backward compatibility, in most cases, you can reduce the downtime to the dataset and server upgrades. This can make a significant operational difference where HP TRIM is now considered to be a critical system.
With the above in mind, a mixed environment provides an acceptable level of functionality for the period of the rollout, but it cannot: provide the full functionality of the latest release provide the same degree of stability of a single-build implementation.
Mixed environments are not suitable for either testing or training purposes.
At my organization, we're upgrading some clients to 7.1 in order for them to access a 7.1 test environment. Those people seem generally able to access the 6.2.3 production environment without any critical issues, although an undetermined set of features don't work right (ie. modifying Global Settings).
This is to be expected when in a mixed mode. The new client is backwards compatible, but many features will not work properly because of schema changes, moving things out of the registry and local files, etc. Mixed-mode is only meant to be temporary during the upgrade process.