You were, at the time of the update, still a part of Div1 and had all the revenues and expenses of Div1. Since only after the offseason you dropped to Div2, you became eligible to the Div2 numbers, including the salary floor.
Just for the sake of the discussion, let's consider that we do make the change you're proposing (paying numbers of your future league). First, it would require figuring out where you'd end up (which league) before the actual promotion/demotion of the offseason and using numbers for that league. Hard, but not impossible to do. However, that would mean that the knife cuts both ways and teams that are promoting should also be eligible to numbers of their future leagues, and it would mean the'd have to pay the salary floor of the new division even though he's never played in it, got promotion money nor benefited from the higher division revenue boost. You see the problem here, then.
One last thing: the salary floor is closely related to the TV money revenue (as explained in the game manual). Since TV money is not paid "upfront", like the player salaries, it doesn't make sense to change it before the actual promotion/demotion. And if TV money doesn't change, salary floor doesn't change. Consider it as if a part of the contract you'd have to sign to compete in a league: you agree to receive the TV money with a salary floor clause. The risk of paying the salary floor on the last economy update of the season if you demote is ultimately yours.
P.S. Sorry for the long post.