If you modify the loan (and don't replace it) no new disclosures are EVER needed....
There are always exceptions in Reg. Z (that's what makes it so much fun!) and the exception here comes from the OSC--Section 226.20(a), Comment #3:
3. Variable rate.
i. ....
ii. 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 either:
A. Increases the rate based on a variable-rate feature that was not previously disclosed; or
B. Adds a variable-rate feature to the obligation....Notice that Subsection ii(B) this rule is triggered by the addition of a VR
feature and would apply even if the the resulting VR is lower than the FR it replaces.