I think the biggest difference is less about the assignment’s format and more about the level of thinking expected of you.
With CIPD Level 3, you’re building your foundation in people practice, so the assignments generally focus on showing that you understand key HR principles and can apply them appropriately to workplace situations.
At Level 5, you’re expected to go further. Rather than just explaining a concept, you’ll normally need to analyse situations in more depth, consider evidence, compare possible approaches and justify why you would recommend a particular course of action.
It’s also worth mentioning that Level 5 doesn’t necessarily mean writing a thesis. Both qualifications can be completed through written assignments, but the complexity and level of critical thinking increase as you progress.
So if you’re moving from Level 3 to Level 5, the biggest adjustment is probably learning to move from “this is what the HR concept means” towards “this is how I would apply it, why I would choose this approach, and what evidence supports my decision”.