I'm not sure what "having the entire plot planned out from the beginning" really means, though. Eliezer ends up retconning things relatively frequently, so I wouldn't be surprised if he had a plot point like "Voldemort captures Harry" followed by "Harry escapes", but with too few details in between to make the logic ironclad.

If HPMoR was a conventional book, then Eliezer would have a lot of time to edit it and make all the retcons behind the scenes -- even fairly major ones -- but it isn't, so he can't.