Can an employer take away accrued vacation time in Wisconsin without any notice to employees?

I have a question about accrued vacation time in Wisconsin. The company I work at has approximately 25 employees, all but two full-time. We receive a certain number of paid vacation days on the anniversary date of our full-time hire. Our employee handbook (written decades ago) states that one week can be carried over to the next year, though this has NEVER been enforced in the 15 years I have worked there. We were notified on April 30, 2015, after some employees questioned HR, that all accrued vacation over one week was wiped out as of January 1, 2015. We never received any formal notice of this happening, therefore, many were unable to use vacation time that they had accrued. One example, an employee who had been there 20+ years, with at least nine weeks vacation accumulated prior to his anniversary date in September (so, he just received four additional weeks), and he lost approximately 9 weeks of vacation time that he was given no option of using up. My question is, since our vacation is accrued based on the work anniversary date and not the calendar year, shouldn't each employee be given the option to use their accumulated vacation time prior to their anniversary date in this current year, even though they are claiming that the change was effective 1/1/2015 (on the calendar year)? I would think that if one had vacation accrued in excess of one week at the beginning of the calendar year, and their anniversary date was not until later in the year, then they should be allowed to use that vacation time up until their anniversary date, and any excess over one week accrued at that time would be lost. (We are an at-will workplace, with no signed contracts.) We WERE informed in late January 2015 that we would no longer be able to carry over sick days, though that was retroactively dated to 1/1/2015 as well.