what is the difference with existing Mouj class?
Many differences.
CIWS systems are different between Jamaran (Fath) and Sahand (Kamand). Air search radar also different between the first and second ship. Jamaran seems to be fitted back with the Asr Radar and Sahand retained the Plessey one. The Fajr-27 firing system in Sahand was a IR/EO stabilized system and in Mowj it has a FCRadar. And the main SAM system was just two Sayyad guided missiles without real FCRadar for mid-course guidance (it has radar just for final approach) in the case of Jamaran while Shahand was fitted with a navalised version of Mersad-16 SAM system with real FCR.
Sahand propulsion is also different because is equipped with 4 diesels 5.000 shp engines while Jamaran was equipped with two large 10.000 shp diesel engines.
Dena Mowj layout it is much more similar to Jamaran with the Fath and Fajr-27 guns. It has also just to coffin for 2 sayyad missiles, but in the case of Dena at least with the indigenous WM-28 FCR it would possible to guide the Sayyad. Anyway Dena has a different radar that seems not to be the Asr radar and has been build in aluminum at ISOICO.
Zagros seems to be a completely different warship, besides the hangar for an AB212 it has two large masts that is equipped with ESM/ECM system and probably ELINT systems making she very capable for intelligence gathering. The propulsion system while unknown or at least we don´t have any evidence, it would be probably formed by 4 diesel engines, with funnels split at the sides of the ship similar to Deylaman just for allowing a hangar for the helicopter. The small size of the hull and the high superestructure seems to not allow fitting anti ship or SAM systems.
The most advanced of them it is Deylaman which seems to be equiped with a phased array system with 4 antennas and 8 canisters for surface to surface missiles. She would combine the more advanced Kamand CIWS and the best radar of all iranian fleet and has a large space in the foredeck that would allow even a small VLS system.
Incoming Taftan seems to have similar layout to Deylaman.
As you can see Mowj class has reached a bottleneck and IRIN needs a bigger warship capable of combining most of those systems (phased array radar, hanger, VLS and ESM/ECM equipment at once). But Mowj has been a real school for engineering and navy personnel. It is a real step towards a moder navy, with some of them with digital systems and advanced missiles and electronics.