Comment 16 for bug 1823740

On the topic of mutable bindings, there is no solution currently except for further development in Juju core. Regarding this condition which can affect clouds where default bindings, or any bindings are in play, the work-around is now documented in the charm-guide release notes for the 19.04 OpenStack Charms:

https://docs.openstack.org/charm-guide/latest/1904.html

tldr; Add a 2nd (new) memcached application instance to the model and define the bindings at deploy-time for that new application. See link above for more detail.