Disassembling the gearbox shafts is necessary to replace the shaft parts.
Removing the gearbox is discussed in the article - "How to replace the gearbox of the Niva Chevrolet"
Disassembling the gearbox is discussed in the article - "Repairing the gearbox of the VAZ-2123 car"
The Chevrolet Niva family of cars (VAZ-2123) are equipped with 4-cylinder engines with in-line vertical cylinders and an overhead camshaft, equipped with a distributed fuel injection system.
Replace hoses and pipes if they are damaged