I would strongly recommend upgrading the management to R81.10 first. As mentioned, R80.40 goes end-of-support soon.
Once you have the management upgraded, I recommend using ISOmorphic to install R81.20 on each firewall, downgrading them in-place to R80.40 (before the first-time wizard, using 'installer clean-install'), running the first-time wizard or config_system, installing jumbo 197, setting up your bonds, then using vsx_util reconfigure to rebuild the members. Plan on a total outage for all traffic through the cluster. There may not be one, but assume there will be.
The reconfigure process is the same process you would use to replace a failed cluster member. Shut down one old member, replace it with one new member, and boot the new member. Once it is running, 'vsx_util reconfigure' on the management establishes SIC, makes some configuration tweaks, then pushes all the VS configuration to it. It might be able to sync with your old member, or it might not. Assume it won't be able to. Once the new member is up and everything on it looks good, apply any local modifications like dynamic routing. When you're done, shut down the second old member. The new member should recognize there are no other members in the cluster anymore, so it should take over the VIPs. If everything works through the new member, repeat the process with the second new member.
Then once you have swapped the hardware, R81.10 management servers have the ability to push upgrade packages to firewalls. Use that to upgrade the cluster from R80.40 to R81.10. This can be a separate window, but it should be as soon after the hardware swap as possible.