Existing network settings for existing VMs will be preserved on recompose operations, as you've seen. The feature you want to use is the ability to assign multiple network labels to a pool, introduced in 5.2. Network labels are then explicitly allocated after provisioning and refit operations. You can change the pool settings to have the new network label set, then do the recompose operation. See VMware View 5.2 Documentation Library for more details.
Mike