I bought my Phantom Vision +2 in May, downloaded the Software Assistant (v. 3.0) and did all the calibrations, but I elected not to upgrade my firmware. I also enabled NAZA mode, which I have used exclusively since my first flight.

Now, I've decided to upgrade the firmware. When I connected the Phantom using Software Assistant v.3.0, no message appeared indicating a new upgrade was available (I'm still currently using firmware v. 3.0). Under the "Upgrade" tab, there are no updates available.

I have now unistalled Software Assistant 3.0 and reinstalled Software Assistant 3.2, but there are still no upgrades available.

Do I have to switch back to Phantom mode before I can upgrade, or is there something else I should be doing?

Thanks

Use Assistant v3.2.
On the Upgrade Tab
Click on the entry in the Firmware Column or Upgrade Column and see if it does not offer you an upgrade...