I'd suggest performing a snapshot of 3150, downloading it off appliance for possible recovery operation first.
Then, "migrate export" and "migrate import" to the virtual R77.30 instance deployed from ISO with sufficient resources.
If you can, retain same IP and hostname of the virtual appliance and maybe, NAT it behind another VM to avoid conflict.
Proceed to upgrade using CPUSE in virtual environment to see if the process will result in any error messages.
If it will, try working those out in virtual environment.
Use snapshots before each attempt for VM, in case you'll need to repeat the procedure or to try something different.
Once you'll get through the upgrade and verified that all is well, not only you'll have easier time going through it in production, but you will have an upgraded virtual instance that you may be able to fall back to, should something go wrong with Smart appliance.
Good luck!