Here is the summary of the events regarding "The Melancholy of Haruhi Suzumiya 2".

July 7, 2007 The second season of "The Melancholy of Haruhi Suzumiya" was announced in Asahi newspaper.
December 18, 2007 The official website announced that the second season was canceled and they will produce a "new" animation.
February 4, 2009 In a TV ad, the cover note of Newtype magazine March issue read "A TV anime of Suzumiya Haruhi will be aired from April".
February 6, 2009 Newtype magazine March issue reported that they will reair (Aratamete, meaning once again, all over again) "The Melancholy of Haruhi Suzumiya" in April. Director Takemoto Yasuhiro said "The production of the new season is underway."
March 9, 2009 Newtype magazine April issue reported that the "reairing" will start on April 2nd. Kadokawa will also stream the anime on Youtube from April 15th.
The summaries of the interview with the VAs are below:
Hirano Aya "This year will be full of Haruhi. I'm pleased to play Haruhi again".
Chihara Minori "The Melancholy of Haruhi Suzumiya finally starts! I'm wondering how Haruhi will entertain us again."
Goto Yuko "I'd like to thank the fans who have been waiting for a long time. Let's enjoy the anime together again!"

Source: Screenshot of the TV ad of Newtype March issue
Source: Scan of Newtype April issue
Source: Scan of Newtype April issue 2

Updated==========
According to the TV programs guide on Yahoo Japan, the first episode of the "reairing" is "Episode 00 Asahina Mikuru no Bouken", which is the same as the first airing.

Source: Yahoo TV Guide