Dear ludvigb,

--Does this only apply to new iSCSI volumes created?
All existing will remain the same

--Is upgrade recommended and without consequences for the existing volumes?
Yes, we recommend to have the same release, even though different releases will not create any problem. Also, please make sure to save configuration and settings before perform the update. (Maintenance->miscellaneous)

--If use of File I/O is so important when hosting ESX/ESXi volumes, this should have been mentioned in the quite detailed white paper "What are the best practices when it comes to VMWare ESX Server 3.5 and Open-E?"
This is mentioned in a pop-up window that shows every time users create an iSCSI volume.
Also, in our Knowledge base there is an article explaining that, under title "What are the best practices when it comes to VMWare ESX Server 3.5 and Open-E?"
https://kb.open-e.com