Which contract does the employee need to honor

Your boss' verbal promise regarding benefits is not particularly helpful by itself, but it would be very significant if after you went part-time the company continued to offer the benefits you enjoyed as a full-time employee. Your position is strongest if there is any written evidence, such as deductions for employee participation in benefit plans shown on pay stubs or evidence of payments actually received from the company after your status changed. The legal principle in action here is the ratification of an agreement by course of conduct or pattern of behavior. In other words, if there is a question about whether there was an agreement, and yet the parties consistently acted in accordance with the terms of the disputed agreement, the course of conduct or pattern or behavior is probative of the validity of the agreement and a court is likely to find that the disputed agreement is valid. The bottom line is that no, the company does not necessarily need to honor your original written contract, especially if it was an essential term of that contract that you would have to work as a full-time employee to receive the benefits in question, but if the company continued to offer benefits even after your status changed, you may have a case. It's also important whether other employees in your situation continued to receive benefits even after their status changed from full-time to part-time employees. Obviously, your case is stronger if others got benefits and weaker if they didn't.