That's an interesting point of view. And here's my understanding: You get 5GB free storage with the free plan. And you get 50GB paid storage with the paid plan. Plan is upgraded, storage is replaced, and data is moved.
After all, you can't have a free trial and a paid service at the same time.
You pay to get brought up to a 50GB total package.
You are 100% entitled to a refund on your 5GB free when you leave that plan.