Banks
I think everyone has hit on the point but I'd like to be a little more specific. You have established every month a "Trip Swap Bank". No you will not find a reference to this any where in the contract but the "intent" is in there. (I forget where but I could find it with some searching) The bottom line when SWAPPING with open time is you will be using this "swap bank". As mentioned previously, you can go up to -12 CH in that bank every month, and the bank is reset every month. If you drop and pick up via make-up, it has no effect on your swap bank.
For example, you have a 60 CH month. You drop a 35 CH CDG trip. Your make-up bank is now increased by 35 CH. Later you see a sweet 40 CH trip that is all DHs and you want to swap your 6 CH O&B for that. You cannot. You are swapping a 6 for a 40... which will exceed the 12 max swap overage you are allowed. Even though the NET effect to your BLG is a -1 CH from BLG as awarded, that is not how the computer is programmed. In order to pick up the 40 CH trip you will have to drop the 6 CH trip and M/U the 40 CH trip.
When dealing with trip swaps you must remember it is a separate bank and cannot be mixed with other banks.