You normally have 20 minutes (I think) to edit your posts. Within that time there is an edit button next to the quote button, but it doesn't "stop the clock" -- you have to be finished and save before the time limit. After that you have to contact a moderator or administrator to unlock it so you can make any changes you require.
If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------