Overtime for salary

I was hired two years ago by my current employer. I received a letter confirming I was hired at a specified salary but I would receive overtime for any time over 46 hours. The overtime would be paid quarterly. I am in Purchasing and not in a managerial position. Last week the owner announced the company will no longer pay overtime to salry employees and he expects us to work as many hours as it takes to get the work done. I have worked 50-60 hours over the past two years just to stay current. Last week my boss spoke to me about increasing my work load by 5 to 7 hours a week. I explained I did not want to work anymore hours regardless if I got overtime or not. I have stated this several times in the past. They are really pushing me to work more hours and I think I could loose my job over this.
Can they just decide not to pay me overtime even though I have it in writing? Can they force me to work more hours or fire me because I don't want to? They are not asking other people in the company to work this kind of hours.