Can an at-will employer change my hourly rate on my final paycheck that was sent to me after I left?

I worked for a family owned business for about four months. They are an at-will employer and I quit without notice. I was a manager-in-training and had been told by the owner that during my training, my pay would be bumped up, though this never occurred and I was still being paid at my previous rate. The service my employer used to communicate schedules and other issues with employees showed my pay rate as the higher rate the owner had told me, as well.
After I quit, rather than having me come to a business location to pick up my final check, the owner asked for my address and mailed the final check to me. When I opened it up, my hourly rate had been changed to minimum wage. These were hours I had already worked before putting in my resignation, under the general agreement that I'd be getting at least the rate I had been getting.
When I said I would report them, the owner told me that because they're an at-will employer, they're perfectly allowed to change my rate to minimum wage. I thought this only applied to hours not already worked. Are they right? Was it legal for them to switch my rate for my previously worked hours as retaliation because I left without notice?