Making a field read only while in a draft mode for all the sales and purchase documents


I’d like to make a field called ‘Item Code’ static/read only while it’s in a draft mode as shown in the screenshot below,


Is it possible to make this field read only across all the sales and purchase forms by changing any settings under Customize Form?
I am hesitant to make any changes to the actual code and therefore seeking an advise to achieve it via front end.

In the transactions, Items are selected based on item code. Also, can you explain the need for this?

The need for this is, many times accidentally our new staff under training had wiped down the Item Code and populated some other items code instead when for instance a sales order is opened in a draft mode. And honestly there is no need in our business workflows that Item Code field should be editable anyway.