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I think it’s easier to modify Heroes III into Heroes V than vice versa.
Here are some links I can suggest:
Heroes V: Armageddon's Blade – the mod isn’t ready yet; the goal is to transfer the atmosphere of Heroes III to Heroes V. However, I’m not sure if this will affect the graphics. But they definitely won’t make the graphics two-dimensional.
Heroes 3: Hota – if you have played through the regular Heroes III extensively, then here is an unofficial but very high-quality and extensive add-on.
Heroes 3 WOG – even more extensive, but in my opinion less high-quality and more colorful unofficial add-on for Heroes III.