Personally, I would leave the original figures alone if you included your property improvements in the total structure price already being depreciated. The cost of the professional help you would need to separate the old heat pump out to deduct the remaining depreciation not yet taken would result in a deduction that would reduce taxable income. But it will result in a tax reduction that is far, far less than what it would cost you for the professional help to do this correctly. On top of that, since rental property almost "always" operates at a loss on paper tax time, it would have absolutely no impact on your 2019 tax liability what-so-ever. Not one penny.
Then just add the new heat pump as another rental asset that gets depreciated over 27.5 years.
