I am going to answer this as it is in the US.
First most states employment is at will and you could just be fired if you don't agree with the terms. But the chances are that your company doesn't want to get rid of their staff and you.
If their entire goal is to turn their business into a sweatshop and work you to death because you are salary then the probable outcome will be the staff end up finding better jobs slowly since their hourly average will be much below industry standard.
What would I do if I faced this situation? Call a team meeting with the people being effected and the management. I am almost positive your co-workers are not happy about this and have the same stance as you. You need to ask management why the change.
If your shop has tons of OT and they want it to continue for free as a team I would ask for salaries to be compensatory of the average paycheck you are getting now. It is really that easy. If you work 50 hours now and go to salary your salary should be based on the 10 OT hours a week you will work, and there will be an expectation that you continue working those hours or become so efficient that you can do 50 hours work in less time.
Now if management says that "things are changing" and they don't expect the hours... well obviously I wouldn't believe it. But this is really easy to counter too. You ask what the expectations of work are for salary employees. Then give examples of what would happen if a salary employee works 10 hours more one week. If this doesn't include a chance for the employee to recoup their time then you are back to square one of getting a lot less for same hours. At my work I am salary and have to work nights and weekends to do things out of peak hours - loading new software, server cleanup, and whatever. My boss could care less if I left at 2 PM a couple days during the week.
The probability of this working for you lies in your coworkers standing together. If it doesn't work people will leave at a rapid pace (we did this in our US offices with our network team and it failed miserably). Good luck.