The system (1) has a first change unit (10) in which two or more friction coupling elements are brought into or out of engagement and a second change unit (12) which can perform the change in at least two higher and lower stages arranged in series. A gear change abnormality detector detects an abnormality in a change involving bringing the two friction elements (C2,B1)in the first unit into engagement. A gear change blocking unit for the first change unit blocks a change involving the engagement or disengagement of the two friction elements in the first unit if an abnormality is detected.
