From what I understand of the grace period, all revisions get wiped. This would imply there's no real way to confirm whether or not a post was changed and therefore whether you should be able to remove your vote or not.
One solution would be to keep the first version of a post; you will then know if it's been edited within the grace period.
If the first version isn't kept then there is a column,
LastEditDate in the
Posts table. From what Nick describes of how the re-indexing of posts for search takes place the time a post was last edited is stored. If it's edited in the grace period you could store this value in the
LastEditDate, even if you don't store the revisions themselves.
I suspect this column is used to create the edit timestamp link beneath a post, which might cause problems with links being available but no versions being visible. If you didn't want this to appear you could use the edited timestamp described by Nick to create a simple flag, to determine whether the post was edited during the grace period and allow people to revoke their votes if this is true.