Epic and La Grandes Battles expansions differences

Hi all,
I am new to the system - just preordered base game and most expansions.
I ordered the Epic expansion.
But I fail to see in what it is different from the Grandes Battles: both have Epic and Grandes Battles boards, for an example.
So can one explain me what The Grandes Battles adds to Epic?

Thanks in advance.

Don't know all the particulars, but tentatively, The Grand Battles (Expansion 7) adds 12 EPIC scenarios, 12 Grand Battle scenarios, a mounted Grand Battles board, two extended pre-printed grand battle maps for battles of Borodino and Waterloo, new rules and blocks for leaders of honor (may order more units on a take command card), iconic leader powers (for Napoleon, Wellington, and others), a few rules tweaks, and additional other stuff such as units, terrain, etc.
Note: you will need expansion 5, Generals, Marshals, and Tacticians, to effectively use the EPIC and Grand Battles expansions.

Thank you for the answer.
I find strange that two types of "larger" battles were created...

differences:
LGB (see 2023 pdf, swift changes in blue):
-LGB map with 6 hexes more
-Corps commander have cards, if does not receive a Command card from the CIC, he must choose to play one of his Command cards from his hand.
-Important Note: During a turn when a Corps commander does not hold any Command cards in his hand, and does not receive a Command card from the CIC, the Corps commander may not order any units that turn. No initiative rolls here in LGB.
-Squares. The card come from the hands Corps commander. When a Corps commander does not have any Command cards and the Corps commander wants a unit to go into square, he must ask the CIC for help. The CIC may refuse or allow one to be drawn from the CIC’s hand.
- Both if they play a card, draw one card each. Then the rack.