What's the essential mechanical difference between these two standards?
Are there minor mods you can make to switch them?

Induction motors run slightly slower than the synchronous speed they
are designed at. For example, the windings can be designed to make the
stator's magnetic field rotate around the rotor once per 60 Hz cycle.
In one minute at 60 Hz (60 cycles per second) the field rotates around
the motor 3600 times. The rotor, which consists of bars running sort
of parallel to the shaft, has current induced in these bars (that is
why they call it an induction motor) which reacts with the spinning
magnetic field to produce a force, and thus a torque.

At "no load" (a freely running motor) this induction motor will spin at
nearly 3600 rpm. When load is applied the motor will slow down
slightly. At the rated load the motor will slow to its rated speed,
something like 3500 - 3550 RPM.

An induction motor rated at 3550 RPM is designed with two poles in the
stator (the windings on the outside of the rotor) such that the
magnetic field rotates around the rotor once per 60 Hz cycle.

An induction motor rated at 1750 RPM is designed with four poles in the
stator such that the magnetic field rotates around the rotor once every
2 cycles of the 60 Hz.

There is no way to convert 3550 RPM induction motor to run at 1725 RPM.

There is also no way to run it at 3650 RPM unless you use an electronic
speed controller that can increase the frequency of the power driving
the motor.

DJS