We had an outside ac unit replaced and the ac company that installed it, changed my dual zone inside to a single zone. Now my downstairs is hot and the upstairs of the house is freezing cold. In the winter it's vice versa the downstairs barely gets warm and the upstairs is boiling hot, how can this be corrected and why would they change it in the first place. I have had several ac companies look at it, and can't seem to fix this problem.

The original contractor needs to make it right. If they say you didn't require two zones then they are at fault.

Hopefully, they didn't remove or toss the dampers and control panel.