Whether or not you sign anything may not matter. That is unless your employer's action violates the terms of company policy, an employment contract, union agreemnt, etc., then it is legal. Additionally, this treatment must not stem from some form of actionable discrimination. The fact is that in an "at will" employment relationship (and most are), an employer can sert the terms and conditions of the workplace much as it sees fit. This includes pay reductons and bonus plan restructuring. While you cannot be made to sign anything, you can be terminated for this. In fact you can be discharged for any reason or no reason at all.