When attempting to upgrade your Heroku Redis 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.
In order to upgrade your Heroku Redis 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 Redis instance. If you have maxed out the number of connections for your Heroku Redis plan, Heroku cannot connect.
If your Heroku Redis instance is very heavily loaded, the standby lag may be too great for us to be able to upgrade your Heroku Redis 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 Redis before upgrading your Heroku Redis plan.