I am assuming you will make the changes in ECC correct ?
Once you do so there are standard reports in SRM side which can be used .
BBP_LOCATIONS_GET_ALL - Transfer all plants from all backend systems as locations
BBP_LOCATIONS_GET_FROM_SYSTEM - Transfer all plants of a backend system as locations
BBP_LOCATIONS_GET_SELECTED - Transfer selected plants of a backend system as locations
Once you transfer SRM will have a business partner created for each plant and this BP number is used in ur shopping cart.Now you need to assign this in your user attributes in pposa_bbp ..
BBP_LOCMAP table List of all plants you can see ..
To delete the existing plants if they do not get deleted automatically not sure if they will . .
use either of the belwo options :
1. FM BBP_BUPA_EVENT_DELE2
2. Transaction bbpmainint
3. FM BUPA_DEL or FM BUP_BUPA_DELETE
Hope this helps !