Not just resistance heating. Frequency conversion is not necessary for many applications.
If there are pulleys and belts, or sprockets and chains, the mechanical drive ratio can be adjusted to accommodate the different motor speed.
Many machines can simply be operated at the higher or lower speed if the voltage is appropriate. (near-constant volts-per-Hertz ratio)
But are you sure you have 120/208V 50Hz? That's a very unusual service configuration.