Have you posted your character profile in
this forum yet? Since you've already been sorted with the character you've signed up with, your best bet would be to either post your modified profile in there or to edit the profile that you originally posted.
Don't worry about making modifications to the history, appearance and personality of your character. People add stuff and make modifications to their profiles all the time.
The workshop forum is designed so that people can both give and receive feedback when modifying their profiles. If you'd like someone to critique, that's the best place to go.
I don't have access to the admin CP to verify this, but I'm pretty sure that only people from the usergroup of "Muggles" can post there. The purpose of the Sorting Ceremony is to place people in their Houses.
If you'd like someone to look through your history and profile before you post, you can ask one of the staff members, probably one of the SMOOSHers just because they always seem to be online. If you can get it to me in the next day or two, I can look over it myself. =)
Alternatively, if you want your character to be re-sorted (ie, if you're planning to do a complete revamp and think that Roderick might not fit in Slytherin), you should PM one of the admins (Sundari Harmony, Cloe Carrigen, Tycho Brynhild, Vorian Atreides or Blyssenor Wright, all profiles linked from the sidebar) and ask them about re-sorting.
I hope that answered your question.