Comment 10 for bug 1468871

The ground work for this has been done in the ssh migration re-work done during the 1907 cycle. The main issue to watch for is to ensure that during upgrade the charm 'flattens' the ssh keys across all relations. One way would be to delete all the existing knownhosts and start again, but that might take a long time on a large cloud ... which is probably the target for this bug.

With the 1907 charms the knownhosts are cached (but may be disabled). I think that an appropriate upgrade strategy will be to switch on the caching in the existing charm, then do an upgrade to a version that will include this bug/feature, to minimise the upgrade time.