You'd need to tell them everything. Essentially what you're doing is reconstructing the design and development process, not the mistakes and iterative phases, just the lessons learned, so someone else can do, as much as possible, what you would be doing if you were updating the project. Don't destroy URLs. The only acceptable excuse for link rot is going out of business. It kills me that professionally-staffed, million dollar content sites routinely change their entire file structure, or delete articles that are a month or two old.
News sites do it, software sites do it.
Good, let's remove it. Let's rename it. Let's get them to perform a Boolean search. I hate this. It's beyond stupid.