NY 12C itself never changed after 1930; however, the routes it overlapped with did. In the early 1940s, NY 5S was truncated eastward to downtown Utica and replaced with NY 69 from Rome to Utica and NY 5A between Yorkville and Utica. NY 46A, meanwhile, was renumbered to NY 274 and truncated to Holland Patent in the early 1950s. NY 365 was assigned and overlapped NY 12C between Stittville and Barneveld until it was truncated to end at NY 12C north of Stittville in the late 1950s. On January 1, 1970, the NY 12C designation was eliminated. From Utica to Whitesboro, NY 12C was supplanted by the routes it once overlapped; however, NY 69 was truncated to NY 5A in Yorkville. East of Stittville, NY 12C became part of a re-extended NY 365. The remainder of NY 12C's former routing was redesignated as NY 291. Reference markers for NY 291 north of NY 49 still read "12C".