Never mind. It finally updated the extended replica automatically. It just took a LONG time.
How to Add Disk to Hyper-V Extended Replica
Hi. I need help with Hyper-V disk replication. I added a new virtual disk to a VM and used this command to update the 'disk set' to update Replication so that the new disk is replicated to the Replica Server from the Primary.
Set-VMreplication "SERVER" -ReplicatedDisks (Get-VMHardDiskDrive "SERVER")
from this article: https://social.technet.microsoft.com/Forums/windowsserver/en-US/9e57084e-45d5-414a-971e-07524591bc22/hyperv-replication-remove-disks?forum=winserverhyperv
This added the new disk to the replication but how do I also add it to the Extended Replication Server? The same command didn't work on the Replica Server.