The term "modification" is not used in Regulation Z and therefore is irrelevant in determining whether the proposed post-consummation event will trigger new disclosures. To be relevant, an event must rise to the level of a "refinancing" (as defined by Section 1026.20(a).)
"A refinancing occurs when an existing obligation that was subject to Subpart C is satisfied and replaced by a new obligation undertaken by the same consumer." Will your proposed modification result in satisfaction and replacement of the original note with a new note?
"Even if it is not accomplished by the cancellation of the old obligation and substitution of a new one, a new transaction subject to new disclosures results if the creditor...adds a variable-rate feature to the obligation." Since you are removing, not adding, a variable rate feature, this additional test (found in an Official Interpretation of Section 10236.20(a)) does not apply in your case.
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...gone fishing.