If the line of credit is the only change then we would do a modification (change in terms). However the change in terms would need to take place before the HELOC matures. Once it matures you will have to treat it as a new plan.
https://www.bankersonline.com/regulations/12-1026-009#c(c) Change in terms. (1) Rules affecting home-equity plans. (i) Written notice required. For home-equity plans subject to the requirements of §1026.40, whenever any term required to be disclosed under §1026.6(a) is changed or the required minimum periodic payment is increased, the creditor shall mail or deliver written notice of the change to each consumer who may be affected. The notice shall be mailed or delivered at least 15 days prior to the effective date of the change.
The 15-day timing requirement does not apply if the change has been agreed to by the consumer; the notice shall be given, however, before the effective date of the change.
https://www.bankersonline.com/regulations/12-1026-0402. Changes to home equity plans entered into on or after November 7, 1989. Section 1026.9(c) applies if, by written agreement under §1026.40(f)(3)(iii), a creditor changes the terms of a home equity plan—entered into on or after November 7, 1989—
at or before its scheduled expiration, for example, by renewing a plan on different terms. A new plan results, however, if the plan is renewed (with or without changes to the terms) after the scheduled expiration. The new plan is subject to all open-end credit rules, including §§1026.6, 1026.15, and 1026.40.