upgrade form grizzly to havana

upgrading swift can be done safely, just make sure you read the release notes and also the changelog to make sure there are no new configuration parameters that are required. Also, unless stated in the release notes, there is no need to restart the main swift services although some package providers already do a restart on post-package installation.

You basically want to perform the upgrade slowly, first do it on one storage node and make sure everything is ok by checking the swift logs. Once you are sure everything is running fine and you can perform some direct API requests against that storage node that has been upgraded. Then go ahead and proceed with the upgrade. If you have 5 zones, do one zone at a time and once you are done with the storage nodes on all zones proceed to the proxy servers and so on.

If you are using openstack-keystone, make sure that the version you are using will work with openstack-swift (havana)

https://wiki.openstack.org/wiki/ReleaseNotes/Havana#OpenStack_Object_Storage_.28Swift.29

https://github.com/openstack/swift/blob/master/CHANGELOG

I should have mentioned there that you just need to reload all the swift services if a restart is not required.