The Improvements were made before I started renting the property when I was living there
Then the cost of that improvement should have been included in the cost basis of the property as a whole, when it was originally converted to a rental 10+ years ago, and would therefore not be listed as a separate asset anyway. If this was not done, then I would leave well enough alone.
To do it now would required you to file a form 3115 - Change in Accounting Method, in order to add those items and account for 10+ years of depreciation you did not take. Not only would the cost of this be high, but the 3115 is not simple by any measure and requires professional help. That professional help is not free either, of course.
