The main change is probably to do with lists rather than rules. Arab Conquest infantry seem destined to be classified as Irr Bd (I) rather than Wb (S). I don't know the reason for it, but I suspect it's to do with better knowledge of how they fought and the fact that the Battle of Yarmuk (against a Maurikian Byzantine army with a lot of Bd (I)) took all day to win.
As to whether that would affect your army, I don't know. Nothing has changed in how you assign PIP dice to irregular armies, and you can make Irr Bd (I) advance in a straight line as well as you can make Irr Wb (S) do so. However, where Wb (S) will spontaneously enter combat and fill gaps in the line, you'll have to pay PIPs to make the Bd (I) do this. On the up-side, the Bd have a higher combat factor, and about as many quick kill opportunities as the Wb.
If you used Khalid as a brilliant general, you could probably get away with two commands. He gives you the chance to double his PIPs in vital turns (such as when you want to get a line of Bd into contact who've been broken up by bowfire).
