It's really pointless to change this now. Typically, a new central air unit would be classified as residential rental real estate and depreciated over 27.5 years. But apparently, what you have was classified as an appliance, and therefore depreciated over 5 years. If it's fully depreciated now, you're best to leave well enough alone and move on. Besides, depreciation a new A/C unit which probably cost less than $6K is not going to have any appreciable impact on your tax liability if depreciated over 27.5 years. In fact, it probably won't make one penny of difference in your tax liability.
With the unit fully depreciated now, if you have to replace it in the future (typically 7-10 years) when you show disposal of the depreciated one, there will be no deduction for it, since it's fully depreciated now. But the new one would be correctly classified as rental real estate and depreciated over 27.5 years.
