The SU-122M was an attempt to modernize the SU-122. The new version had a new fighting compartment and a new main gun, the 122mm U-11. The U-11 used the same ammo as the 122mm M-30S. Ultimately, the modifications made were very expensive to manufacture, and they overloaded the chassis. The project was scrapped.
After the failure of the SU-122M, Uralmash decided to once again attempt to modernize the SU-122 on the concept of mixing in new design elements that the newer tanks used, like the SU-85. The SU-122-3 was mostly identical to the SU-85, except for the 122mm D-6 main gun and a few small changes.