Lock the Original Estimate time field

[Pankaj Sharma]

We want Users to estimate the 'Original Estimate' time when creating a task and this original estimate value gets auto-saved in a custom field 'Estimated days'.

If they can change it mid way through the workflow, we will still should be able to compare it 'Estimated days' value. 

I am wondering how it can be achieved, shall I use ScriptRunner plugin.