Only restriction at Flybe was that after converting to the Q400 pilots had to do 100 hours on it before they could be rostered to fly the 200/300 and the 400 in the same duty.
I did it quite often and it was no big deal, except that after getting out of the 400 into a 300 you were constantly checking whether you had left the gear or flaps down because the climb performance was so appalling in comparison
