First of all, please check the spring which is holding the ball in the holes of the shifting gate. If the spring gets loose, the shifting is soft and a gear can even fall out. In such a case you have to replace the spring. This is the most common problem with the shifting.
Best Regards Petr Dolezal, VARI Export dpt.
Yes, you have to put the gearbox to the side. It is also recommended to lock the gears with the pin (the pin used for locking gears 1 and R for ploughing).
It is necessary to drain the oil and open the gearbox, the ball could damage the gears. Then return the ball to its place, check the spring and replace if necessary. At the same time don't forget to replace all the gaskets and sealing rings damaged during the disassembling/assembling process.