Steps to reproduce:
- Set a light type to "Area".
- Set the shape to "Disk" or "Tube"
- Set the light type to something else than "Area"

Expected:
Light mode and shadow settings are freely editable

Result:
Light mode is locked and shadow settings are limited to the light mode

Reproduces with:
HDRP master & Unity 2021.2a11 ; HDRP 10.7 & Unity 2020.3.19f1

Note:
Can not fully reproduce with Unity 2019.4 & HDRP 7.x, as the light mode field is not grayed out in this version, allowing it to be changed after having changed the type. I suspect that this was overlooked when graying out the field depending on the area light shape was introduced.