Unwinding a brushed (AC, 120v 60hz) motor to the tune of one turn, what for to gain length
There is no delicate electromagnetic balance, especially in brushed motors. The logic is simple: there were (for example) 200 turns, you removed 2 turns. This means 1% reduction in turns. By the first approximation, the motor will run 1% faster, the stall torque will rise by 1%, and heat output will go up up by 1%. You are likely not going to notice either of this.

Note: these calculations are approximate and are only valid when the change is very small; if you remove, say, a quarter of total number of turns, you will need to perform a much more complex calculation.