Are Variable Motor Windings Possible?
Brushless motors requires electronic control circuits that time the switching of the excitation with the rotor position like a DC motor commutator. Alternatively, the excitation can be timed to simulate a polyphase AC power supply. Anything done to change the number of winding turns that are energized would not seem to simplify or improve the performance of the usual modes of control.

DC motors with commutators including universal motors can be configured for multiple speeds in this manner. AC induction motors can be controlled this way also, but only to a limited extent. This is one way to configure a motor for a multi-speed fan. A more complicated switching arrangement can be used to change the number of poles to make a two-speed or perhaps even a three-speed motor.

This sort of thing has been done with electro-mechanical switches. To do it electronically would seem to add nearly the cost of an electronic speed control to offer only two or three fixed speeds.