_template is used to create new deployments. once a deployment has been created, modifying only _template will not change the newly created deployment. _template is also used to create the deployment used for an upgrade, so it does need to stay current w/ your config.
blasadmin -a affects only _template, and any other actual appserver deployments - eg 'default', server-job, etc. it does not affect _spawner, _launcher or _pxe.
so if you need to make db changes, you will have to run blasadmin -s <deloyment name> to touch _spawner and _pxe (_launcher doesn't have any db connections) and blasadmin -a to affect _template and your other deployments.