Depends on where you upload.
LMA - easy to change at any time.
Dime - if you load the info file only into the Info File field, you can change it easily without hosing the torrent.
eTree - don't know of a way to edit the torrent without having to start over
In general, you can't modify the files at all after starting to seed via BT. For BT sites, I usually include what I know in the torrent, and then just update the comments section of the listing to reflect updated information like set list, etc.