Back dated promotion or not means nothing with regards to incentives, the effective date is all that matters. According to your last post I re-affirm that your incentive should kick in 30 May 08. See your Orderly Room! Pilots often have their own criteria's in early DP's until he/she receives her/his Wings, and only by verifying your Pers File could I make a firm determination.
Vern, yes he would go to the equiavlent or next higher incentive (in $ value) to where he is and yes would begin receiving incentives from there ... based on what we know. That is to say unless there are other issues relating to his situation which we are not in the know about. Only his OR could sort this out by confirming the information on his Pers File.