Special charm upgrade procedures

The OpenStack charms are designed to accommodate every supported series and OpenStack release wherever possible. However, upgrades may occasionally introduce unavoidable challenges for a deployed charm. For instance, it could be that a charm is replaced by an entirely new charm on the new series. This can happen due to development policy concerning the charms themselves or due to reasons independent of the charms (e.g. the workload software is no longer supported on the new operating system). Any core OpenStack charms affected in this way will be documented here: