Sorry for the delay, but I was out of the office most of yesterday -- this is the first that I've seen of your question.
A commission policy is an implied employment contract, because when you perform in reliance on the terms of the policy, you are thus entitled to compensation based upon that policy -- under the legal doctrine known as "contract by promissory estoppel." So, your boss is wrong here as a matter of law -- your signature is not required to make a contract, concerning your already earned sales commissions. All you need is a copy of the policy and proof that you were paid under that policy in the past.
However, once a new policy is set forth in writing, if the employee continues to work for the employer after having notice of the policy, then a new contract by promissory estoppel is created, and the employer and employee are bound by that new contract.
As far as what you as an employee can do, the answer is, regrettably, that under Labor Code 2922, your employer can terminate your modify your employment "at will," at any time, for any reason, or for no reason at all -- except for discrimination based upon race, color, nationality, ancestry, religion, creed, sex, sexual orientation, pregnancy, equal pay between sexes, age or disability -- or, where you are terminated for following a well-established public policy (e.g., jury duty, report of employer's alleged criminal activities to law enforcement, etc.).
BotXXXXX XXXXXne, if you don't like the commission plan, and you don't see unlawful discrimination or public policy violation, then dust off your resume and move on, before your driven crazy.
Note that any change of employment contract which effectively reduces an employee's wages by at least 20% would permit the employee to quit and still obtain unemployment benefits. Anything less risks a denial of UI benefits.
Hope this helps.
NOTICE: My goal here is to educate others about the law. I am always available to answer your follow-up questions after you click Accept – however, if you do not click Accept, the website gets paid, and I receive nothing. This is true, even if you are on a subscription plan. So please click Accept, so that I will be able to continue to provide this service for others in the future.
And, if you need to contact me again, please put my user id on the title line of your question (“To Socrateaser”), and the system will send me an alert. Thanks!