I am at will employee and my boss will change me from management to a class reducing my rate of pay can I file for unemployment?

Probably. The State will determine if you are eligible for unemployment.

Yes, you may file a claim for unemployment. Any one may do that. But, will you win? Likely not. GA DOL would have to find that you were downgraded substantially, and that you were done so through no fault of your own. And that a reasonable person would have quit this job under these circumstances, rather than finding another job and then quitting. GA DOL rarely does that.

Call your state's unemployment commission and ask them. You are saying that this is a constructive discharge, but it will depend on your state's interpretation of that.