They are changing the rifle on the statue because the artist accidentally gave him a Nazi rifle.
During World War II, the Sturmgewehr 44 assault rifle used by German forces made a deep impression on their Soviet counterparts.The select-fire rifle was chambered for a new intermediate cartridge, the 7.92×33mm Kurz, and combined the firepower of a submachine gun with the range and accuracy of a rifle.In July 1943, an earlier model of the Sturmgewehr was demonstrated before the People's Commissariat of Arms of the USSR. The Soviets were impressed with the weapon and immediately set about developing an intermediate caliber fully automatic rifle of their own to replace the PPSh-41 submachine guns and outdated Mosin–Nagant bolt-action rifles that armed most of the Soviet Army.