If they add the DLC-content for free in the expansion, you do not "rebuy" it. By buying the DLC before, you had the opportunity to use it before the Expansion.
It would really simplify the system if a new expansion would automatically include all DLCs so far (and any Deluxe Features). Now you already have 8 separate DLCs (including Azteks and R&F) to activate/deactivate.
Of course, I'd be rebuying it. They would use those civs as a selling point for the new expansion and people would pay for them. If all that I bought was a license to use the new civs a year earlier than others, then they would be free now. That's not the case, which means that they still have monetary value and would thus factor into the price of the expansion.
Arenanet had this problem with Guild Wars 2. They released an expansion, but bundled it with the base game and then sold it for full price. Why should I rebuy the base game just to get the expansion? Forget it.