Company changed me from salary to hourly.

My manager suddenly informed me today that I'd be meeting with HR (I already knew something was up) so I did. Turns out they are moving me from salary to hourly effective immediately, I've been salary for over 3 years and my last raise was almost 2 years ago. The company is doing well, I'm hardly absent and do my best to fulfill my duties so it feels like a low blow and a step back. I told my gf about it and she mentioned they might have needed to give me a 30-60 day notice, does anyone know if they are allowed to do this without a notice?