As I understand it power transformers generally (always?) have turn ratios that exactly match their nominal voltage ratios.
Control transformers, for whatever reason, are "compensated" to adjust for expected voltage drop. When used in reverse the output voltage will be low by twice the expected voltage drop.
But that does not mean that power transformers can always be used in reverse. (NEC 2014 prohibits that when the manufacturer does not explicitly approve).
Among the potential issues are extremely high inrush currents when the "wrong" winding supplies the magnetizing current, and high hysteresis losses on other than nominal input voltages because the voltage adjusting taps are on the wrong winding.