I totally missed that while reading your post yesterday, sorry about that!  
 
Not really any "gotchas"  that I can think of with 9.3.   But there are some out there in general,  for example.  upgrading ONTAP 9.1 to 9.3 they had users decrypt the their disks during an upgrade.    
 
I would also test connection to the external key mangers during boot up too before you actually encrypt the disks.  
 
And review the power guide before you start doing it.  https://library.netapp.com/ecm/ecm_download_file/ECMLP2572742