Found some good PowerCLI Examples | ForwardOrReverse including the one I was looking for, to move VMs from one datastore to another, one at a time:

# Storage VMotion ALL VMs from one LUN to another (One at a time)
Get-Datastore "Old_Datastore_01" | Get-VM | ForEach-Object {Move-VM -VM $_ -Datastore "New_Datastore_01"}

The nice thing about this is you can open multiple powershell windows and do a few migrations concurrently, if your storage infrastructure is capable of it, without kicking off movement of all VMs at the same time from the source datastore (get-vm | move-vm would cause this to happen).