You won't have new capabilities such as Vault Cache until you upgrade, but you do not need to upgrade all EV 2007 clients at once to EV 8. And if you are fairly liberal with who gets Vault Cache (as in all workstations and laptops instead of just laptops), it probably will be in your best interest to roll out the new EV 8 client in phrases. Vault Cache is now processed on the server side rather than the client side under EV 2007's "Offline Vault". A phased rollout will help to avoid having your mailbox archive server(s) be overwhelmed with initial Vault Cache requests. Vault Cache is CPU intensive.