If I'm missing something, please do say. But for example, I tried to swap out Robben -- the game says that won't be effective until Matchday 2. I try to switch my bench substitute, same deal.
Unless you're playing something wholly different.
Also, you're allowed only 4 transfers per matchday, and they would (along with your choice of captain and substitute) have to be completed by Wednesday morning. And I'd rather wait until then before picking my Matchday 2 transfers.