Yes you can upgrade without losing your data and your volumes. If you have NAS-R3 Enterprise version and want iSCSI features you would need to purchase the License Key to upgrade your NAS-R3 to DSS.

When doing this please make sure that you are on the latest version of NAS-R3 Enterprise (5_0_up46_build_3088) and save your settings in Maintenance > Miscellaneous and select all options for Setup and Configuration as well as save locally.

To find out where to purchase the License Key please click on the link below to find a Distributor or Reseller near your location.

http://www.open-e.com/where-to-buy/