Create an SNMPv3 item. Do its "mass update" to change type from SNMPv2 to SNMPv3. Observe snmpv2-item.png.
There are two problems here. The first is that mass update does not clear fields irrelevant to the new item type. However, this is a major problem and is probably not going to be solved anytime soon.
The second problem, though, which is fixable in the short term and should be fixed anyway, is that fields irrelevant for the item type should be hidden in item configuration.