User blog comment:Jimeee/Skyrim 1.9 beta update adds Legendary difficulty and raises level cap (Updated Apr 8th)/@comment-99.187.238.244-20130405154913/@comment-67.61.249.237-20130405175409

Yes, I'm pretty sure you don't loose any learned spells when you legendary one, because that wouldn't make sense and it'd be too easy to work around (save the spell tomes until after you go legendary).