It is very common to install steam turbines when there is a steady HP steam flow through let-down valves to a lower pressure header.  The power savings usually justifies the turbine installation and maintenance costs.  Turbines usually incur more maintenance cost than electric motors.
 
The steam turbines can drive air-compressors, pumps or any other drive applications.  Air compressors and pumps are the most common steam turbine drive applications in process industries.
 
Based on the quantity of steam flow, pressure and temperature conditions of steam at up-stream and down stream of the let down valve, the turbine's shaft power output can be calculated.  If there is enough steam flow at the let-down valve, your option of replacing a 250hp motor is technically feasible.
 
There is no need to change the air compressor, but a gearbox may be required as turbines run at a higher RPM than the motors.
 
Turbines are variable speed machines, while AC motors are constant speed drives. Depending upon the air compressor's control needs, the turbine controls could be specified.