They weren't digitalized. They turned their organic essence and such into a fine powder that, if everything had gone correctly, would have been reformatted back into the individual Precursors. Unfortunately, the technology that would have done this malfunctioned and the ships carrying the powder were later found by ancient humans. After running some tests, the powder was deemed harmless and humans began feeding it to a domesticated species known as Pheru. After several generations, the Pheru began to have strange growths appear on their bodies, which other Pheru would proceed to eat. Around the same time, abortions/failed pregnancies became commonplace in the Pheru. It wasn't long after this that the Flood became as we know it today.