CPFF Modification

We have a USG Cost Plus Fixed Fee contract (Completion form) for a specific scope of work.  The contract was awarded 3 years ago and will be complete the in the 12 months or so.  Upon execution of the effort, it was determined that a specific work scope was inadvertently left out of the TINA proposal and not included in any BOEs. In other words, it was not proposed at all.  Unfortunately, this work scope must be completed in order to accomplish the overall required deliverables of the program.

The customer has been informed of this and has requested pricing for the added scope.  Would this be priced as a contract modification by the means of a proposal submission for the added scope, or would this be handled as a change proposal of some kind to the original proposal from 3 years ago?  To me, this is a straightforward new proposal for the added scope and the customer would award through a contract modification to the existing effort.

Also, the original proposal was over the TINA threshold, however this would be the first modification and the value of this modification is below the TINA threshold, so TINA would not apply, correct?

Thank you!