Keep in mind that R80.10 management requires significantly more memory than R77.x management does.
On a standalone gateway and Full HA scenario, this means your gateways will need, at bare minimum, 8GB of RAM.
More is recommended if your appliances support it.
That said, your options for Full HA aren't all that different than any other upgrade.
You have two choices:
1. In-place upgrade with CPUSE
2. Migrate export / import ("Advanced" upgrade)
Personally, I would opt for a migrate export/import upgrade.
There are, of course, more steps here and I'll see if I can get someone to provide some more context.