I found this document from Fujitsu. It seems that the Q700 is not included in their transition plan. https://sp.ts.fujitsu.com/dmsp/Publications/public/CELVIN%20NAS%20Transition_V3.0_en.pdf
I tried it on the QE807. First, I downloaded and installed Fujitsu_to_QNAP_0.3.qpkg, and it changed my QE807 to a QNAP NAS model.
Then I was able to update it with QNAP firmware. After that, I followed the rest of the steps to reinitialize my NAS. Of course, I backed up all my data beforehand. Now, my NAS has become a TS-431P and is installed with the latest firmware.