Thanks, EthereaqlN, that's good to know.
This, from an SC2 website....
Yes, replay files are very small. A few hundred KB at most for a longer match. Since none of the artworks, models, maps etc. need to be stored in a replay it doesn't take a any more space than a StarCraft: Brood War replay needed.
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Just to be clear, I should buy the disk Lock On: Modern Air Combat, install it, then download Flaming Cliffs II and use that download to upgrade Lock On: Modern Air Combat? I found Lock On: Modern Air Combat, but it seems to be a rare bird these days.