That’s not actually the same question. Your question was about upgrading the firmware of a USB controller. This question is about upgrading from a 500-series controller to a completely different 700-series controller using the NVM backup and restore functionality in zwavejs2mqtt.
It sounds like node-zwave-js will support this use case in the future, as long as the 500-series controller is on Z-Wave SDK v6.61 and up. Prior versions are incompatible. If your Gen5 stick is on the latest firmware, which has a new SDK version, it should support it. Not sure when it will happen, but it sounds like “soon”.
From an end user perspective there isn’t much benefit to moving to a 700-series controller (at least not yet). If you have an older Gen5 stick and upgrade the firmware, you’ll get SmartStart, which is the biggest differentiator between 700 and older 500 series. So don’t feel like you absolutely need to move unless that’s something you want to do. If starting from scratch, definitely go with a 700. Most of the 700-series improvements are in the end devices (range, battery, performance, etc.), so you still benefit even with 500-series controller if you purchase those devices.
BTW, I would not rule out node-zwave-js supporting OTW controller firmware updates in the future either, it just doesn’t support it right now. After all, you can upgrade the controller firmware in Linux with minicom and xmodem, so I’m sure it’s technically possible.