Through Newton, U.S. 321 was originally signed over Main Avenue. Around 1950, 321 was rerouted to the west over Westside Boulevard, which continued into Conover as Fifth Street Southwest. Main Street through Newton had been co-signed as N.C. 16 along with 321; it remains signed as 16 today. 321's days through Newton, however, are numbered; the new 321 freeway will avoid it. 

In the late 1950s, Lincolnton was bypassed to the west. The original route was probably never signed as a "Business" route. But the "bypass" is now part of N.C. 155; see below. 

In 1961, 321 was extended west of Boone into Tennessee. It was signed over U.S. 421 between Boone and Sugar Grove, and then was sent west into Tennessee along what had been N.C. 603. 

The road in Gaston County was widened to four lanes between the S.C. line and Gastonia in 1969. Around 1974, a short stretch of 321 north of I-84 was widened to four lanes as well. 

321 in Caldwell County was originally signed over today's U.S. 321A. A bypass was built first around Lenoir in 1940 or 1941. In 1955, the Lenoir bypass was extended all the way south to the Catawba River, and the original 321 became 321A. This Caldwell County "bypass" 321 was widened to four lanes in 1969, save for a short stretch north of the Catawba River which didn't get widened until 1981. 

In the 1990s, the 321 freeway between Gastonia and Hickory was built. Around 1994, when the freeway had been completed as far north as the Gaston/Lincoln county line, the old road was designated as N.C. 155 (but in Catawba County, where the freeway hadn't yet been finished, the old road was still signed only as mainline 321). On December 7, 1998, the 321 freeway was finished all the way to I-40 in Hickory (exit 123).[1] 
 

The original plan for the old U.S. 321 -- including the stretch through Newton and Conover -- was to remove the U.S. designations and number the old road through Lincoln and Catawba counties only as N.C. 155. Residents opposed the loss of their U.S. highway, however, and the state agreed to number old 321 as Business 321 instead.  The renumbering (and re-renumbering) didn't happen overnight, though. For a few months in late 1998 and early 1999, the old route sported a mishmash of mainline 321, Business 321 and N.C. 155 signs. In late December 1998 the author discovered one spot in Lincoln County where it was possible to exit the 321 freeway onto the older ("Business") 321, only to find that road signed also as mainline 321 (without a "Business" panel). Then after driving a few miles on this 321-that-isn't-321-anymore, you'd cross the freeway again only to be told you're actually on 155 (sign at right)!

N.C. 155 sign  in late December 1998

This nonsense ended by mid-February 1999. 155 was gone, and Business 321 designations appeared over the length of the old road, including through Newton and Conover.

In the mountains, west of where it leaves U.S. 421, U.S. 321 is rather twisty and less modern than most other U.S. highways in the state are. 

Many 321A signs are of the narrow two-digit style, rather than the wider 3-digit style. In all such narrow signs (right) the lettering is very tall and skinny, and a hyphen is used. While many of these signs have been around for years, several brand-new narrow "321-A" signs were observed by the author in February 1999.

For a few years in the mid- to late-1990s, 321 was signed as N.C. 155 (or was to be signed as 155 -- it partially depended on where you were, and even then signs were inconsistent; see comment above). Locals wanted to keep their U.S. highway, though, so 155 was scrapped in favor of the Business 321 designation. 

For a couple of months in late 1998 and early 1999 both old (Newton/Conover) and new (freeway) 321s had "mainline" status, which is to say the state took its time putting "Business" signs up on the older 321.