Bidding question.

I'd like to swap a spillover trip that has 10 more hours in January than the trip I currently have.

The problem is that if I get it and they use the time of the new trip to factor legal lines for January, it will jack up my January bid.

What trip would they use to figure my legal line value; the one I had when bidding closed on the 12th or the trip I have when they do the actual run (which I would imagine would be sometime tomorrow or the next day).

Thanks in advance.