What is your workload on the existing cluster? You can convert the 2552 to a switched cluster fairly easily (https://library.netapp.com/ecm/ecm_get_file/ECMP1140536) - I'd personally schedule an outage for this, but it is not purely necessary. Once the systems are in the same cluster, migration of data \ SVMs \ LIFs are a fairly easy task and non-disruptive. If you are buying all new disks, or at least enough capacity temporarily before swinging shelves between heads, this is likely the approach I'd use.
Depending on the workload, given that you are seemingly replacing the 2552, you could also look to host-based tools like storage vmotion to move your datasets. Great for VMware environments; less so for CIFS \ etc.