Can my company decrease my paid vacation time that I negotiated at the time of my job offer?

I received an offer for my job in January 2010, at which time I tried negotiating a higher salary but my boss, stating that his hands were tied in terms of salary, offered me more vacation time. Company policy at that time stated (roughly) that employees who have been with the company for 0-3 years get 1 week of vacation, 3-5 years get 2 weeks, 5-7 get 3 weeks and greater than 7 years get 4 weeks. My boss offered me 3 weeks instead of 1 week and explained that it is actually a great perk since I would be in the same category as employees that have been with the company for more than 5 years. I had another job offer at the time that offered a higher salary but when considering paid vacation, my current job was clearly more attractive so I took the job and started in February 2010.

My company just switched over to a Paid Time Off policy starting January 1, 2012 whereby all employees get a lump sum of paid time off days (instead of broken down into vacation, sick days, discretionary, etc). Although this is better for most employees, I am worse off since I am now lumped into the 1-4 years employee category and no longer get the perk that I negotiated when I started. Even though I have not taken, nor do I plan on taking, very much vacation (~5 days in all of 2011), I feel as though I should be eligible for the 4-6 year category (additional 3 paid days off); in principal, I do not think this is fair since I relied on this perk to my apparent detriment as I decided to forego the other job offer based solely on the additional paid vacation.

Is it legal for my company to do this?