2. You can not maintain snapmirror DR replication between 7-mode and c-mode systems.
3. You can install c-mode dataontap on your old storage frame, for that you need to perchase cluster license and other licenses.
4. All your data will be wiped if you do that.
5. It is better to purchase 2 new controller heads and perform transation to that.
6. You can use copy based and copy free transition from 7-mode to c-mode to migrate all the volumes. Apart from that you can also use cluster snapmirror transition to perform single volume migration from 7-mode to c-mode.
Copy based : It uses snapmirror technology to migrate the data from 7-mode to c-mode.
Copy free : We will perform some configurations and later we will swap the disk shelves from 7-mode to c-mode controllers.
Storage Lead | C3i Healthcare connections
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Agreed with the others here, OSSV is your main issue as there is no equivalent with ONTAP now. If you can host the OSSV elsewhere (or use another solution), you'll want to request a Cluster Base license from your partner/rep (it should be free as long as you're under a NetApp Support agreement still). That said, I've attached a cheat sheet that shows the details on how to set ONTAP to boot to 7-mode or Clustered ONTAP - I can't find the original reference, but again your partner/rep should be able to point you in the right direction should you need more help with the details.
Also, for reference the FAS2xxx series is NOT supported for Copy-Free Transition; the explaination I was given is that the engineers writing the procedures and doing the testing didn't realize you could convert a FAS2240-x to a shelf originally and since new hardware has been released a few times since (such as your FAS2552 and the new FAS2650), I suppose they expect smaller customers would still be better off just purchasing a new system and using copy-based transitions.