How you would allocate these old costs to your new fields depends on how you have previously allocated the costs.
If you have been using an icon called "Overhead" and have farmed each field with the icon once and then have allocated all costs to the icon, then all you have to do is farm the newly rented fields with the overhead icon.
If you have previously allocated individual expenses to fields then you will need to go to the Allocate Inputs box (from the Enterprise menu) and select each expense and allocate a negative amount to all of the fields that you previously allocated to. You will then allocate a positive amount to all fields (including the newly rented fields).
Note that any costs that were allocated to equipment will be charged to the fields that they are used in (both old and new).