The appliance has differences between 3.3.x & 3.4. No upgrade path exists, the migration will only migrate configuration settings, not historical data.
If you follow the user guide, it will help you deploy and configure a new appliance. This is the best method.
You will need it running for a full month to collect data, and the old appliance will need to be backed up/archived off for contractual reasons it needs to be kept for 2-3 years.