Currently "Original Estimation" field cannot be edited if any work is logged against the issue.

An option of allowing to edit this field after work logging would be expected.

Business case:
Let's assume that work was started and logged against the issue, and than requirements are changing. Original estimation cannot be updated but in real is different than entered with previous requirements. Than, when you want to compare real time spent on issue vs. estimated - there is no sense to do it.