"this month if all goes well," reveals a Blizzard community manager. It's more-or-less an automated process and needs little input.
In fact the pre-expansion patch (2.0.1) will actually contain "all of the expansion data" from the game disc, however expansion-only content won't be playable until March 25th.
The Crusader, Act V and Adventure Mode therefore won't become available prematurely, and only owners of the Reaper of Souls expansion will have access, obviously.
"The exact date that the predownload will start is still TBD, but we're targeting this month if all goes well. Note: this download will begin automatically (meaning, you shouldn't need to do anything special)," Lylirra.
"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."
"Caveat: Expansion-only content like the Crusader, Act V, and Adventure Mode won't be playable until March 25th and only players who purchase Reaper of Souls will have access to them."
Diablo III: Reaper of Souls releases on PC and PS4 March 25th.