Airgus is quite correct.
In assymetric conditions, minimum control speeds reflect aerodynamic forces available (roll and yaw) to counter assymetric roll/yaw moments.
The assymetric roll/yaw moments are a function of the thrust/power being produced by the propulsion units(s).
As altitude increases, the thrust/power being produced is less, so the yaw/roll moments are less. Therefore less aerodynamic forces are required and the the aeroplane may be flown at reduced speed to counteract these moments.
There are at the moment four questions in the CPL/ATPL question database on this subject.
JP