Don't worry, even if you loose the relegation series, you won't change leagues. You won't ever go lower than Division 5 USA.
After tomorrow's home game, if you are in a relegation series, you'll play the first game of that on Tuesday. You'll be able to play a scrimmage on Thursday. If you aren't in the relegation, you'll only play the Thursday scrimmage.
If you're in relegation, you'll have the 2nd of 3 games on the following Saturday. If needed, the 3rd game will be that SUnday. If not in relegation, you'll automatically have a scrimmage scheduled.
On the Monday monday, the seasons will change, the draft will happen, and your current players get one year older. You'll get a scrimmage that Thursday, and then the first league game of the next season happens the next Saturday.
Does that help?