Slot fees and moving schedules between aircraft of the same time

[Harrit]

So I've been experimenting with handling traffic between c and d checks, and the length of time the plane is out of service.  I notice that when I use the "move schedule" button from one aircraft and select another aircraft of the SAME TYPE, I have to repurchase the slots all over, however it seems if I clear the schedule of the routes first, then select the new plane, and assign the routes to the new plane, it doesn't charge me new slot fees, however my route image and my number of fares go way down for a few weeks

Can someone explain this mechanic and why the need to charge me for slots I've already paid for.  None of the timing changes on the routes whatsoever.