MRP Firm & Unfirm basic question

I understand that MRP will create unfirm jobs based on demand/forecast. Say I then firm a job that is supposed to run at the end of the week for a quantity of 100.

Then say a new order for 200- comes in with a due date that an increase in the quantity of that could satisfy. But the job is now firm.

Is there anyway to get MRP to suggest a change to the quantity versus create a separate unfirm job which could easily end up requiring a separate setup and be very inefficient?

Take a look at the on line help Reschedule In/ Out parameters on the part planning options.

You can use this to control expedite suggestions.

Does that apply to changing the quantity on a firm job versus due date?

Have you looked at the planning workbench, this is where all your job suggestions would be such as expedite, increase, decrease, postpone etc