Are we able to update the SOW on the current change order and incorporate into the NTE/change order mod or is a separate SOW/attachment required?
In order to answer your question, we must ask "Is the upcoming requirement that is urgent within the scope of the original contract?" If yes, then you would need to look at 10 U.S. Code § 2326.
10 U.S. Code § 2326. Undefinitized contractual actions: restrictions
(e) Modification of Scope.—The scope of an undefinitized contractual action under which performance has begun may not be modified unless the head of the agency approves such modification as being—
(1) good business practice; and
(2) in the best interests of the United States.
So, if it is in scope and the head of the agency determines it to be a good business practice and in the best interests of the Government, you would need to document all of that before you proceed. I would also recommend that you run this by your legal department to make sure it complies with the rules regarding undefinitized contract actions contained in 10 U.S.C. § 2326. The FAR and DFARS are silent as to whether or not you must or may update the SOW for a change in scope. Based on the above, I do not see why you cannot update the SOW on the current change order and incorporate it into the NTE/change order mod, especially since it is over the simplified acquisition threshold.