Upgrades to anti-aircraft missile systems typically combine improved missiles, radars, and operator consoles.
Usually missile upgrades drive changes to other components to take advantage of the missile's improved performance.
Therefore, when the Soviets introduced a new S-75, it was paired with an improved radar to match the missile's greater range and altitude.

SA-2A; SA-75 Dvina (Двина - Dvina River) with Fan Song-A guidance radar and V-750 or V-750V missiles.
Initial deployment began in 1957.
The combined missile and booster was 10.6 m (35 ft) long, with a booster having a diameter of 0.65 m (26 in), and the missile a diameter of 0.5 m (20 in).
Launch weight is 2,287 kg (5,042 lb).
The missile has a maximum effective range of 30 km (19 mi), a minimum range of 8 km (8,000 m), and an intercept altitude envelope of between 450 and 25,000 m (1,480 and 82,020 ft).

SA-N-2A; S-75M-2 Volkhov-M (Russian Волхов - Volkhov River): Naval version of the A model fitted to the Sverdlov Class cruiser Dzerzhinski.
Generally considered unsuccessful and not fitted to any other ships.

SA-2B; S-75 Desna (Russian Десна - Desna River).
This version featured upgraded Fan Song-B radars with V-750VK and V-750VN missiles.
This second deployment version entered service in 1959.
The missiles were slightly longer than the A versions, at 10.8 m (35 ft), due to a more powerful booster.
The SA-2B could engage targets at altitudes between 500 and 30,000 m (1,600 and 98,400 ft) and ranges up to 34 km (21 mi).

SA-2C; S-75M Volkhov.
Once again, the new model featured an upgraded radar, the Fan Song-C, mated to an improved V-750M missile.
The improved -2B was deployed in 1961.
The V-750M was externally identical to the V-750VK/V-750VN, but it had improved performance for range up to 43 km (27 mi) and reduced lower altitude limits of 400 m (1,300 ft).

SA-2D; Fan Song-E radar and V-750SM missiles.
The V-750SM differed significantly from the A/B/C versions in having new antennas and a longer barometric nose probe.
Several other differences were associated with the sustainer motor casing.
The missile is 10.8 m (35 ft) long and has the same body diameters and warhead as the SA-2C, but the weight is increased to 2,450 kg (5,400 lb).
The effective maximum range is 43 km (27 mi), the minimum range is 6 km (3.7 mi), and the intercept altitude envelope is between 250 and 25,000 m (820 and 82,020 ft).
Improved aircraft counter measures led to the development of the Fan Song-E with its better antennas which could cut through heavy jamming.

SA-2E: Fan Song-E radar and V-750AK missiles.
Similar rocket to the D model, but with a bulbous warhead section lacking the older missile's forward fins.
The SA-2E is 11.2 m (37 ft) long, has a body diameter of 0.5 m (20 in), and weighs 2,450 kg (5,400 lb) at launch.
The missile can be fitted with either a command-detonated 15 kt nuclear warhead or a 295 kg (650 lb) conventional HE warhead.

SA-2F: Fan Song-F radar and V-750SM missiles.
After watching jamming in Vietnam and the Six-Day War render the SA-2 completely ineffective, the existing systems were quickly upgraded with a new radar system designed to help ignore wide-band scintillation jamming.
The command system also included a home-on-jam mode to attack aircraft carrying strobe jammers, as well as a completely optical system (of limited use) when these failed.
Fs were developed starting in 1968 and deployed in the USSR later that year, while shipments to Vietnam started in late 1970.

SA-2 FC: Latest Chinese version.
It can track six targets simultaneously and is able to control 3 missiles simultaneously.

S-75M Volga (Russian С-75М Волга - Volga River).
Version from 1995.

SA-N-2 Volkhov M-2 naval
SA-NX-2 M-3 (missile V-800, V-760/755) experimental variant, four short boosters front head, like some UK air-surface 
As previously mentioned, most nations with S-75s have matched parts from different versions or third-party missile systems, or they have added locally produced components.
This has created a wide variety of S-75 systems which meet local needs.

HQ-1 (Hong Qi, Red Flag)
Upgrades to anti-aircraft missile systems typically combine improved missiles, radars, and operator consoles. Usually missile upgrades drive changes to other components to take advantage of the missile's improved performance. Therefore, when the Soviets introduced a new S-75, it was paired with an improved radar to match the missile's greater range and altitude.
SA-2A; SA-75 Dvina (Двина - Dvina River) with Fan Song-A guidance radar and V-750 or V-750V missiles. Initial deployment began in 1957. The combined missile and booster was 10.6 m (35 ft) long, with a booster having a diameter of 0.65 m (26 in), and the missile a diameter of 0.5 m (20 in). Launch weight is 2,287 kg (5,042 lb). The missile has a maximum effective range of 30 km (19 mi), a minimum range of 8 km (8,000 m), and an intercept altitude envelope of between 450 and 25,000 m (1,480 and 82,020 ft).
SA-N-2A; S-75M-2 Volkhov-M (Russian Волхов - Volkhov River): Naval version of the A model fitted to the Sverdlov Class cruiser Dzerzhinski. Generally considered unsuccessful and not fitted to any other ships.
SA-2B; S-75 Desna (Russian Десна - Desna River). This version featured upgraded Fan Song-B radars with V-750VK and V-750VN missiles. This second deployment version entered service in 1959. The missiles were slightly longer than the A versions, at 10.8 m (35 ft), due to a more powerful booster. The SA-2B could engage targets at altitudes between 500 and 30,000 m (1,600 and 98,400 ft) and ranges up to 34 km (21 mi).
SA-2C; S-75M Volkhov. Once again, the new model featured an upgraded radar, the Fan Song-C, mated to an improved V-750M missile. The improved -2B was deployed in 1961. The V-750M was externally identical to the V-750VK/V-750VN, but it had improved performance for range up to 43 km (27 mi) and reduced lower altitude limits of 400 m (1,300 ft).
SA-2D; Fan Song-E radar and V-750SM missiles. The V-750SM differed significantly from the A/B/C versions in having new antennas and a longer barometric nose probe. Several other differences were associated with the sustainer motor casing. The missile is 10.8 m (35 ft) long and has the same body diameters and warhead as the SA-2C, but the weight is increased to 2,450 kg (5,400 lb). The effective maximum range is 43 km (27 mi), the minimum range is 6 km (3.7 mi), and the intercept altitude envelope is between 250 and 25,000 m (820 and 82,020 ft). Improved aircraft counter measures led to the development of the Fan Song-E with its better antennas which could cut through heavy jamming.
SA-2E: Fan Song-E radar and V-750AK missiles. Similar rocket to the D model, but with a bulbous warhead section lacking the older missile's forward fins. The SA-2E is 11.2 m (37 ft) long, has a body diameter of 0.5 m (20 in), and weighs 2,450 kg (5,400 lb) at launch. The missile can be fitted with either a command-detonated 15 kt nuclear warhead or a 295 kg (650 lb) conventional HE warhead.
SA-2F: Fan Song-F radar and V-750SM missiles. After watching jamming in Vietnam and the Six-Day War render the SA-2 completely ineffective, the existing systems were quickly upgraded with a new radar system designed to help ignore wide-band scintillation jamming. The command system also included a home-on-jam mode to attack aircraft carrying strobe jammers, as well as a completely optical system (of limited use) when these failed. Fs were developed starting in 1968 and deployed in the USSR later that year, while shipments to Vietnam started in late 1970.
SA-2 FC: Latest Chinese version. It can track six targets simultaneously and is able to control 3 missiles simultaneously.
S-75M Volga (Russian С-75М Волга - Volga River). Version from 1995.
SA-N-2 Volkhov M-2 naval
SA-NX-2 M-3 (missile V-800, V-760/755) experimental variant, four short boosters front head, like some UK air-surface 
As previously mentioned, most nations with S-75s have matched parts from different versions or third-party missile systems, or they have added locally produced components. This has created a wide variety of S-75 systems which meet local needs.
HQ-1 (Hong Qi, Red Flag)
S-75 Dvina