According to this URL: https://sort.veritas.com/public/documents/sf/5.1/aix/html/vcs_admin/ch_admin_vcs_from_cli54.html
the Parallel attribute cannot be changed after resources have been added to the service group.
I noticed that this is for version 5.1, but my guess is that this will also apply to later versions.
This is how I see the next step(s):
Create a new Failover SG with all resources disabled, then offline and disable the Parallel SG, enable resources and online the Failover SG, delete Parallel SG.