In the Amendment do I have to extend the due date for offers received or can I just put a common cut-off time for receipt of proposal revisions in the discussion letter?
If the revisions are minor then you do not have to extend the due date- that is a judgment call of the contracting officer based upon advice and input from the technical folks. You should also consider if someone (like an incumbent) gets an improper advantage by keeping the deadline the same.
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If the revisions are significant, then you should extend the due date.
While you didn't ask, I would be very concerned about changing the SOW AFTER you've determined the competitive range. Given that the requirements have changed, how can you possible know who is in and who is outside the competitive range? A company that had been excluded originally, may allege that you manipulated the SOW to get your favored companies in and exclude your unfavored companies.