Whenever I want to change a date or dates of a playoff series, I delete ALL games for the series and then add all games. Otherwise, if you try to - for example - delete the game on the 17th and add a game on the off-day/19th, then deleted the game on the 20th (to make it the off-day), etc... you might have weird numbering with the GameID field. It's actually been really long since I experienced that issue so I don't recall the details of what specifically goes wrong... but that's why I do it this way...
At any rate, these would be my steps if I wanted, for example, a 7-game series to start on the 18th instead of the 17th (and assuming it was in the HHoAAAoHH format):
1. Delete all 7 games of the series from the schedule
2. Move forward to the 18th (using the date dropdowns in the upper right, below Select League and Actions), then add the first two games of the series, using the selections in the lower left corner, including specifying Playoff Game and 2 Games (Philly @ NY).
3. Move forward to the 21st, then add three games (games 3 thru 5, NY @ Philly).
4. Move forward to the 25th, then add the final two games (games 6 & 7, Phil @ NY).
This should work fine. That said, you *might* run into issues since you are already on the 17th (I would do the steps above *before* I reach the 17th), but you *should* be ok...
And I just want to clarify - to address your last question - that when you ADD a game to the schedule you can select the type of game - Regular Season, Spring Training, Exhibition, Playoff - in the Type of Game dropdown field in the lower left. If you select Playoff, OOTP will recognize it as part of your World Series.
Hope this is helpful
