I suggest a local attorney consultation, bringing with you all the relevant documents--employee policy manuals and all employment agreements. It is most difficult to give a definitive answer without reviewing all pertinent documents and circumstances. Generally speaking, contracts do require some consideration. Good luck.
Answer given for general advice and is not a legal opinion, which would require an analysis of the facts and circumstances as well as the applicable law and regulations.
Generally speaking, contracts require some consideration. There may be instances where you can have a quasi-contract action called promissory estoppel if you rely on a promise to your detriment. With respect to whether the provisions of an employment agreement are binding on employers that will often depend on state law. For example, in Missouri they are not binding, but are in Nebraska.