How to change MySQL InnoDB cluster from single primary mode to multi primary mode?
Any reason why you're not using MySQL Shell and its AdminAPI to set up and manage your InnoDB cluster?

You can create a multi-primary cluster very easily using the AdminAPI. Example:

var cluster = dba.createCluster("myCluster", {multiPrimary: true})


Also, if you already have a running Group Replication group that you'd like to manage using the Shell you can simply "adopt" it:

var cluster = dba.createCluster("myCluster", {adoptFromGR: true})


And since 8.0.14, you can change a running InnoDB cluster from single-primary mode to multi-primary mode "live". The AdminAPI provides a function for that operation:

&lt;Cluster.&gt;switchToMultiPrimaryMode()&gt;


Also, it is possible to switch back a running cluster to single-primary mode:

&lt;Cluster.&gt;switchToSinglePrimaryMode([instance])


And even to elect a member of the cluster to be the new primary:

&lt;Cluster.&gt;setPrimaryInstance(instance)


Please take a look at the blog post that announced those new features:

https://mysqlserverteam.com/mysql-innodb-cluster-changing-cluster-topology-modes-live/

And for further information check the official userguide.

Cheers,

Miguel