Can a company change a commission rate without first notifying employees?

I am sorry for your situation. Technically once there has been an offer and acceptance of a contract one party can not change the terms.  Your questions leaves a lot on unanswered questions on my end and some issues that need to be looked at before any guidance can be given.  First, you say that you filled out an application for the job.  Was the commission that you were told you were going to be paid listed n the application?  Were you job duties and descriptions listed on the application?  what I am trying to see is if the application could in some way be considered a contract and used to prove that the employer breached the contract by paying you less than your agreed upon rate of commission.  Now, if it can not be used that way you can still try and prove that you had an oral contract and that the contract terms were the same for all the employees (there is strength in numbers). You can file a complaint with the department of labor here too. Good luck.