I Was Tasked With Rewriting A Screenplay At My Former Company. I Started From Scratch And Wrote An Entirely New Script That They've Decided Not To Make. Who Owns The Rights?

Looking for anyone with copyright insights. I was tasked to rewrite an old screenplay my former company owned the rights to -- it was basically a riff on an old movie with a pop star instead of a princess. I was to go in and change her occupation and rewrite enough that the original writer wouldn't notice and sue us if we went and made it. Pretty slimy, I know.
Anyway, I thought the script was terrible and I'm a pretty fast writer so I just decided to start from scratch and make the character an athlete. It's still a riff on the old movie, but every single word is original. They budgeted and scheduled the movie, but ultimately decided not to make it. As far as I know it will be dead forever, and most of the people at the company will have already forgotten about it. However, I think there's a fun idea there and I would like to write it as a book. The problem is, I'm not sure if they would or could come after me if there was any success. I'm sure there will be some changes, but I plan on keeping the title, characters, and general plot.
I have a general understanding of copyright laws; however, my situations is a bit tricky. There were no contracts signed in any relation to this script. I actually never even had a contract with the employer. It was literally a handshake deal. I wrote one draft on my personal laptop and there was no development. Technically, I was paid to write it as I was salaried. I could ask if I can have the rights to the script, but at this point I don't want to interact with them at all. Any help would be greatly appreciated.
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