Fallen Enchantress is not an expansion. It's a standalone.
I'ts important to differentiate between the two. Not only because he doesn't have to buy War of Magic but also because the term "expansion" has come to mean "more things" like factions, units, equipment, spells, quests etc....
Which isn't the case here. Frogboy (owner of Stardock who makes the game) says that Fallen Enchantress will be a completely new game in the Elemental universe.
I don't agree with that, but it will be a new 4x TBS.
They're changing mechanics, gameplay systems, upgrading the graphics engine from Pixel Shader 2 to Pixel Shader 3. That's the big things.
They're keeping the lore, story and stuff like that so it will still be the Kingdoms of Altar, Pariden, Tarth and the Fallen Empires of Magnar, Kraxis and Resoln.
A comparison would be to take Company of Heroes and change the resourcesystem to StarCrafts, get rid of squads in favor of single units (like in StarCraft) and make doctrines into subfactions like in C&C Generals Zero Hour.
Then you upgrade the Essense Engine (used for Company of Heroes) into Essence 2.0 which was used for Dawn of War II.
That's how I understand the difference from War of Magic to Fallen Enchantress.