Would this not constitute a change in terms causing a change in terms notice?
Yes, but depending on the timing of that change in terms dictates whether it's a modification or a refinancing which would require all new disclosures. Any modification, regardless at whose request, after the HELOC matures is a refinancing and requires all new Plan disclosures if remaining an open-end credit.
Converting (modifying) from a open-end HELOC to a closed-end HE is always treated as a refinancing regardless of the timing and all the required disclosure requirements of Subpart C to Reg Z are required..