Can a company just reclassify you as part time even if you are getting at least the minimum hours to maintain full time status and then deny you benefits?

I don't understand how a full time employee can be classified as a part time employee without working fewer hours.

What happened?

Is this a union contract subject to arbitration ? Also what state is this because labor laws a very specific to the state administering it. But if your question has a bottom line of can they cut your benefits.. most likely they can.