This should also be broken down in to 2 parts -
1. Data migration
2. Repurpose the shelf.
What ONTAP version is the 2552 running?
What type of data is being moved? (CIFS, LUNs, VMware etc).
These two questions will help quantify what method to use.
NetApp to NetApp migration can be broken down in to 2 methods. (Well 3 technically, but we're not going to talk headswaps here due to your existing gear).
1. Snapmirror or a varient of it.
2. If you have access a set of cluster switches you could have the option to create a temporary 4 node cluster.