I am confused by your statement... Your FD package starts on a specific date.. what do you mean by checking in a day or two before? Do you mean with a different RO reservation? a split stay? if so, your FD starts when you check in for the 2nd reservation.
as a tip if you do a split stay with FD, do your FD first and then the RO part.. This way you have your park tickets for the entire part.. with FD you have to get park tickets.. so let's say your FD is for 4 nights, and you do a split stay with RO, for 3 nights, you book your FD but when select park days, add those other day... ex. package for 4 nights, but a 7 or 8 day ticket.. your tickets are still valid when you check out of your FD room and check into a RO room.. Sometimes you dont have to phycically change rooms, sometimes you do.