How do I make a field conditionally read only?

Summary:

We want to lock down the business unit details so that a HR/manager cannot change business unit upon performing the Change Assignment action.

Content (please ensure you mask any confidential information):

We want to lock down the business unit details so that a HR/manager cannot change business unit upon performing the Change Assignment action. This is what we did intitially, to click 'read only' under the component properties.

However it would apply to all other pages using 'Assignment' function. e.g. Hire an employee / Add a Pending Worker. The page then does not allow any update on Business Unit as well. Moreover, it affects system prompt and causing error in proceeding.