Yes, that's correct. When you upgrade a newer version you can select image with NVE. Going from Non-NVE to NVE should be fine, but going from NVE to Non-NVE could be an issue, there is a BUG:1012881 (But it is already Fixed in 9.2).
NVE is not supported if the command output displays the text 1Ono-DARE (for no Data At Rest Encryption), or if you are using a platform that is not listed in Support details. Also, it needs "AES-NI" capable processors applies to certain old and all newer platforms (FAS2650 is supported).
The following command determines whether NVE is supported on cluster.
cluster::> version -v
NetApp Release 9.1.0: Tue May 10 19:30:23 UTC 2016 <1Ono-DARE>
Which platforms support NetApp Volume Encryption (NVE)?