If I was you, I'd upgrade at the end of the 1st day and then you won't have the problem of the blackout day (Or any time once you are inside the park if you aren't planning on doing Park-to-Park).
I am curious though, if you upgrade on the 2nd day of a 2-day pass, do they credit you the value of a 2-day ticket or do they credit you the value of a 1-day ticket (since that's all that's left on your 2-day ticket).