I highly suggest talking to the PTO (the organizer) about that very soon. If they know that someone doesn't show up, they can decide to put you in instead (don't quote me on it though as I've never seen this situation IRL before).
YOu can't substitute players in after the final standings have been released. The player who got 8th would have had to have dropped before final standings to be released in order for the person who got 9th to be substituted in. I know this because I got subbed in after I got 5th a a BRs.
Also I can't post replys in the Card Questionings area atm, I dunno why. (Hence the profile comment instead of the reply.)