Hi,
Logically I would think this could be achieved by performing the following tasks:
1. Migrate all data from the aggregate you wish to offline/destroy.
2. Connect the new DS4243 w/ 1TB drives and create a new aggregate on the controller you wish to be dedicated to SATA.
3. Perform an NDMP copy of the root vol off of the aggregate with SAS onto the new aggregate with SATA and bring it online. Please refer to the link (https://communities.netapp.com/thread/14353)
4. Offline the empty SAS aggregate and destroy it. Zero all of these disks and remove ownership from the original filer to the new filer and add these disks to the existing SAS aggregate on the other controller. (Would recommend looking up best practices on adding disks/raid groups to existing online/active aggregates).
***This is not guaranteed to work for you in your scenario however I performed this successfully in my lab with an older system running 8.0.1. I would recommend scoping out this project and doing your due-diligence in regards to planning, homework, etc. Also reaching out to other resources never hurts!***