Can a company that hired you at a mandatory 45 hour a week position just tell you that you are now at 40 hrs

Yes, it is entirely legal unless you have evidence that this change was based on illegal discrimination (based on race, age, gender, so on) or you had a contract that say you will always have 45 paid. Your employer can always change the terms of your employment (other than the exceptions I noted), like they did here.

Unless you have an employment agreement that guarantees you a specific work schedule for more than two years, the employer is free to change the position moving forward.

Yes, this is most probably legal. The positions of at-will employees can be prospectively modified by an employer for any reason or no reason at all upon proper notice. However, if you had an agreement for employment that set aside the at-will presumption, then the employer must abide by the agreement. From what you have posted, it is unclear whether or not you had such an agreement in this case, so you may want to talk to an attorney about it. However, I will say that a mere understanding that your current position will have X number of hours is probably insufficient to set aside the at-will presumption. Rather, there needs to be some kind of contract entitling you to the position: for example, a promise to have the position with X hours for Y years, a promise not to alter the terms of your employment without good cause, or an inducement to leave a hirer paying job. I hope this basic information provided some helpful guidance.