text: It was never changed. In order to understand this you have to know first that the Jewish day starts from when the sun sets. in The 14th (before sunset) they put the blood on their doors and in the midnight G-d killed all the first borns (already the 15th because it's after sunset). when the egypcian found out what happened they begged the jews to leave, the jews were in such a hurry to leave that they didn't have time for their bread to raise up properly and that's why we eat the unleavened bread.