Each faction has different upgrades. For Haven, you take all the new units (Rebels), except for Inquisitors. Sometimes armor is more useful than Bloodlust.
For Academy, you take all the old units, except for Gremlins and Titans. For Necropolis, you take all the new units, except for Vampires. For Sylvan, you take all the new units, except for Druids and Archers. For Dungeon, you can take all the new units (I especially like the new lizardmen).
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal