Snapmirror, yes would involve some downtime, but it's the simplest. You can do it without downtime, but you would have to do the following:
Upgrade the 2552 to a newer rev that matches what the 2720 is being deployed at.
Convert the 2552 to a switched cluster using dedicated cluster switches (Something like a CN1610 or BES-53248)
Add the 2720 to the cluster, creating a 4 node cluster.
Once the 2720 is configured, vol move everything to the new system.
Remove the 2552 from the cluster
(And if you want, remove the cluster switches)
This could be helpful as well for you -> https://docs.netapp.com/platstor/topic/com.netapp.doc.hw-upgrade-controller/Miscellaneous%20-%20Upgrading%20controller%20hardware.pdf Starting page 23.
Convert the 2552 to shelf is pretty straight forward.
You will want to be sure that any trace of the old system config is removed.
delete all aggrs.
un-own and un partition all drives.
the boot menu opt 4 or 9a can be helpful with this too.
after that you can just power it down and install the IOMs and connect it to the 2720.