Issue
When attempting to upgrade your Heroku Key-Value Store plan, you receive an error like Your HA standby is currently lagged behind your primary datastore, making some operations unavailable at this time. Please try again later.
Resolution
In order to upgrade your Heroku Key-Value Store (KVS) plan, we must be able to determine that there is not too much lag between the primary and the high-availability standby. In order to do this, Heroku must be able to connect to the Heroku KVS instance. If you have maxed out the number of connections for your Heroku KVS plan, Heroku cannot connect.
If your Heroku KVS instance is very heavily loaded, the standby lag may be too great for us to be able to upgrade your Heroku KVS plan.
In order to resolve this, you may need to scale down (ref: https://devcenter.heroku.com/articles/scaling) your processes that are connecting to your KVS instance before upgrading your Heroku KVS plan.