Are you referring to depreciation on the new property?

It depends on if you make the election or not (1.168i-something).  One way is that you depreciate the new Basis using the new placed in service date.  The other way is that you continue depreciating the old property using the old Basis and old placed in service date, PLUS you depreciate the 'extra' amount for the new property using the new placed in service date.