Stephanie Johnson, Diablo III’s community manager, posted on the Diablo III forum:
“This download will begin automatically, meaning, you shouldn’t need to do anything special,
“Similar to what we’ve done with other games, the pre-expansion patch (2.0.1) will contain all of the expansion data that’s on the normal disc. So, for players who don’t predownload, they’ll basically get everything that’s available whenever they patch up to 2.0.1.
“Expansion-only content like the Crusader, Act 5, and Adventure Mode won’t be playable until launch and only players who purchase Reaper of Souls will have access to them.”
Stephanie Johnson also said the exact date of the preload is still to be decided, but they are “targeting this month if all goes well.”
Diablo 3: Reaper of Souls is going to be released on March the 25th for PC and Mac.