@Jason Smale 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. Accepted answers show up at the top, resulting in improved discoverability for others.
Issue: Unable to delete an instance of "Cloud service (classic)" after attempting to start a migration to ARM.
Solution: Customer shared - "I had to run the "Migrate to ARM" function again through the Prepare step. When the Prepare step fails, I had to click on "Abort". The abort function removed the "migration in progress" lock. After that, I was able to delete the service without issue."
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