Well for one composite made aircraft can achieve up to 20% weight savings in empty weight.
These weight savings could directly be used to carry more payload.
The original turbojets had 93.2 kN thrust and the new turbofan has 118 kN thrust. The aircraft has two of those engines.
The extra thrust would increase the maximum takeoff speed given a length of runway and would increase max takeoff weight accordingly.
Wikipedia claims the Mikulin AM-3 engine has a specific fuel consumption of 93.2 kg/(kN•h) and the D-30KP-2 has a specific fuel consumption of 0.715 kg/kgf hr.
93.2 kg/(kN*h) is 0.914 kg/(kgf hr). 0.914/0.715 = 1.278. So the new engines should have ~28% less fuel consumption.