How to failback to Original Writer instance after Upgrade

Unfortunately, no. The upgrade is designed to failover first to reduce downtime. The readers and write should be on the similar instance class in Aurora cluster. You can try either of these:

• Upgrade your new writer now, which would automatically failover to give your original configuration with both instances to new instance sizing.
• Manually failover the cluster to switch the configuration from console, CLI or API call, and then you can upgrade reader without a failover.

Note that when you perform a manual failover, you can specify which instance to fail over to, although specifying the instance ID is optional:

https://docs.aws.amazon.com/cli/latest/reference/rds/failover-db-cluster.html

What I like to do for Aurora clusters with >2 instances is put 2 of them in priority tier 0, and all the others in some other priority tier (2-15). That way, all the failovers default to switching between those 2 instances in the top priority tier and there's no worry about some other instance with a too-low capacity, wrong AZ, etc. being promoted to the writer.