The Compose action's field becomes read-only after it runs once. In complex flows that depend on long chains that are dependent on each other, I've had to delete and re-create up to 7 Compose Actions everytime I needed to make a small change in one of the ones at the top. This is far more common while using Compose Actions to create more and more complex flows.
You can now edit expressions anywhere in your flow, read more here: https://flow.microsoft.com/blog/use-expressions-in-actions/