My father did similar to this decades ago. The company he had worked for with great loyalty since he graduated in the 1960's was taken over and the new management treated him appallingly.
He couldn't leave due to pension restrictions in those days but they eventually, after making his life miserable for years, made him redundant. He was almost at the stage of a breakdown it had been so badly managed.
He left to start his own company. After a few months they realised that no one else had his skills or knowledge as they had made his team redundant over the years too.
They panicked and brought him back as a consultant. He charged them as he saw fit for his knowledge of the systems he had put in place. We are talking major systems that would have seen three big cities in the UK struggling. Goodness knows why they missed this when they deemed his department not worthy of continued funding.
After the way he was treated I think he had the last laugh.
No one knew his role was so essential, no one listened, no one even wanted to know until after he had left. After 30 years in the same role it was his designs and innovation the company profited from.
Technically his work and notes belonged to the company who took over. They obviously retained the systems as they were installed. They asked him to forward his notes, but my father had a photographic memory and didn't write comprehensive notes only abstract calculations, and as he had more or less been his own boss for decades this hadn't ever been an issue. The calculations were forwarded but made no sense to anyone.
He worked as a consultant until the systems he designed became obsolete, then he took retirement. It was a profitable end to a dedicated career.
I doubt his 1960's employment contract would have had anything about intellectual property rights to procedures. It was quite basic back then.