This would likely be a well documented case if something used to be legit, and then turned into a malicious site. Bans aren't just handed out without doing research so there's definitely going to be some discussion on this point. Lets say that the staff member who issued a ban didn't see any evidence of this being legit back in the day, then I'm sure including this information within the ban appeal will solve the issue.

For example, if you owned a domain, and sold it off at some point, but still have links to it in your posts from when you owned it, and its suddenly turned into a malware site. You should be able to prove that the domain was moved on afterwards via domain records, and that might possibly help the situation. Being able to prove that you used to own a domain, and no longer do would probably result in the ban being removed in most instances.

At the moment there's been a lot of users getting banned for plagiarism that was posted a few years ago now that users have the tools to find this on a massive scale which is a good thing in my eyes. It has already been quoted in this thread, but theymos approves of banning anyone that has plagiarized in their post history no matter how far back.