The sub spindle motor is dual wound and the "gear changes" are accomplished by changing which windings you are using by M code. You will get more torque on the low end by using low "gear", but will have an RPM limitation. The advantages by using this method are changing ranges on the fly, low maintenance and less vibration. Downside - you have discovered it: much lower torque on the low end.
No motor changeout possible that I know of.
What are your cutting conditions? Are you already running in low range?