Yes, the employer may do this unless the employee has not just an offer letter--which only describes what the pay, benefits, duties, etc. are at the time of hiring--but an actual written employment contract "locking in" the benefits. In the absence  of a written employment contract, all employment is employment at will; and one effect of employment at will is that an employer may change compensation, including benefits, at any time, and is not required to foreover compensate the employee the same way or provide the same benefits.