I recommend that you call Raymarine tech support and ask this question. I suspect that you can do a direct upgrade.
We bought a new Axiom Pro 12 RVX in June. It shipped with Lighthouse 3.16.84... I guess it had been sitting on a shelf for a while. It worked well for a couple of months and then began losing the
depth reading and was also spontaneously rebooting. Raymarine suggested that we upgrade to the latest version of Lighthouse (4.7.172) and do a factory reset. I was skeptical, and thought I'd be pulling the unit when we lay up for the
winter and sending it in to be bench-tested under warranty.
Two weeks ago, I backed up all the data, upgraded the software, and did the factory reset. The unit has been working perfectly ever since.