US 123 into Greenville 1947 Greenville County

US 123 on new alignment into Greenville 1953 Greenville County

About 1953, US 123 was placed on a southern bypass of Easley, leaving behind US 123 Bus (now SC 93).

US 123 through Easley 1951 Official

US 123 Business through Easley 1953 Pickens County

Also in 1957, US 123 was given a bypass of Seneca around the north side. The old route became US 123 Business (now SC 59 and SC 130).

US 123 through Seneca 1951 Oconee County

US 123 bypasses Seneca 1963 Oconee County

In 1958 (bridge date), US 123 was given a bypass of Clemson on the north side. The old route became US 76-123 CONN (now S-39-4 east of Seneca River and abandoned routing west of it).
In 1959, US 123 was straightened out from Seneca eastward, avoiding the Newry area. A small piece became part of SC 130, but mostly it became S-37-1.

US 123 via Newry area (note error SC 25 shield) 1951 Oconee County

US 123 on new dual lane routing 1963 Oconee County

In 1961, US 123 was extended east along US 25 Augusta Rd, then east across the new Camperdown Bridge over Reedy Falls to connect with the newly built US 29 Church St at Cleveland Ave. This is implied on Official Maps from 1961-71 and explicitly on them from 1964-69.
In 1964, US 123 was moved onto new freeway from Clemson to connect with the Easley Bypass. The old route through Central, Norris, and Liberty became SC 93 and a piece of the original Easley Bypass on its west end became SC 93 CONN.

SC 13 Georgia to Greenville 1933 Official

US 123 replaces SC 13 1948 Official

US 123 on freeway between Clemson and Easley 1966 Official

In 1965 (bridge dates), US 123 was placed on new alignment bypassing Richland, leaving behind S-37-13.

US 123 through Richland 1963 Oconee County

US 123 bypasses Richland 1967 Oconee County

In 1968 or 1969, US 123 was removed from Pendleton St (became SC 124) and Augusta Rd (remained US 25 Bus which is now SC 20) and instead used newly constructed Academy St to Falls Park to connect to the Camperdown Bridge.
It appears that in 1972, US 123 was rerouted to use Academy all the way around to I-385 Bus as it does today. The old connection to US 29 is shown as primary for years afterwards but I do not know what designation it carried. Today the Camperdown Bridge is gone. No SCDOT map explicitly showed US 123 on either road while they both existed but I know from personal experience that US 123 was on its current routing by 1987. The 1977 Topo does show it on the current routing though.

US 123 extended to relocated US 29 1966 Official

US 123 moves to Academy 1969 Official

US 123 on its current routing through Greenville 2016 Official

Improvements: Fully paved upon designation.
The first piece of US 123 to be 4-lane was in place by 1953: east end of Easley to Greenville.
In 1959, US 123 was 4-laned between SC 130 Seneca and US 76/SC 28 split Clemson.
In 1964, US 123 was 4-laned and freeway from US 76/SC 28 to SC 93 CONN west of Easley.
In 1967, US 123 was 4-laned on the Easley Bypass; also, a couple miles west of the Seneca Bypass; also, a mile or so east of SC 24.
In 1973, US 123 was 4-laned on the Seneca Bypass.
In 1982, US 123 was fully 4-laned from Westminster to Seneca. This brought US 123 as totally 4-laned between Westminster and Greenville.
Comment: US 123 is the major thoroughfare in the foothills of NW South Carolina.
US 123's freeway segment between Clemson and Easley is clearly substandard by today's measures - narrow and hilly.
I have had the adventure of having bicycled US 123 from the Georgia State Line to Clemson, as well as original US 123 (current SC 93) from Clemson to Easley.
US 123 is not very well posted in Downtown Greenville.
C. Mark Sublette provides a summary of the history of this corridor:

Virtually the entire highway route between Toccoa, Georgia, and Greenville, South Carolina, was constructed using the original 1872-73 railroad alignment of the Richmond and Danville Railroad, including the three-span thru-truss bridge over the Tugalo River at the state line. This was abandoned during the Southern Railway’s major rebuilding project of 1916-1918, which company had absorbed the R&D in the great 1894 reorganization by J. P. Morgan and associates. The now-excess right of way was then adapted for vehicle use. Many of the original cuts still exist between Toccoa and Westminster, S.C., but road-widening between Westminster and Seneca, and between Clemson and Central have obliterated the railroad-era cuts. Between Central, Liberty, and Easley the profile of the former rail alignment can still be discerned as long portions remain a two-lane road.