You have probably already done the upgrade, but here is my opinion.
Try to match the number of drives in the RAID sets (10+2, or 14+2, etc) of the 1st SAS shelf, and add drives from the 2nd SAS shelf to the aggregate. This way you will have a similar RAID performance with the original disks within the SAS Aggr.
Since MAX Aggr size has grown very large with current ONTAP, you are well within Best Practices to have an Aggr span shelves.