Can a company change vacation policy without notifying employees?

The company i work for gave 2 weeks of vacation for a year of service. Starting from day one of being employed you accrue vacation time each pay period. You are allowed to go 2 weeks "negative" meaning you can use the vacation throughout the year and earn it back by the end of the fiscal year. Some people used it right away and got paid all 2 weeks, while another 1/2 of us saved it for holiday time. I took 40 hours and then was told the policy had changed and i had none left for the year. Shouldn't the 1/2 that saved it been "grandfathered" into the policy? Thanks in advance.