No, you do not change the original cost basis .
At the "Your asset summary" page for your rental ; little grey button generally to the lower left of screen "add an asset" click it ,
After you click add an asset screen
And add the total cost of the improvements on date placed in service.
"Describe This asset"
Select "Rental Real Estate Property"
Continue
Next screen:
'Tell us a little more About your rental asset"
select "Residential Rental Real Estate"
Continue on out and in put data for a 27.5 year depreciated asset.
Improvements are according to IRS regulation a 27.5 year property.