I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue:
Unable to disable Registration campaign says Failed to Update Policy
Solution:
Under the policies, I had set the migration to complete. Once it's in that state, you can't change the registration campaign. I've set it back to 'In Progress' and I can disable the Registration campaign.
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.
I hope this helps! Thank you again for your time and patience throughout this issue.
Thanks,
Navya.
If the answer is helpful, please click "Accept Answer" and kindly "upvote" it.