I think it's good to work on changes, rather than ripping things apart to start from scratch - you learn more and it's more rewarding when you see the difference. You might find that the algae resolves gradually with new lights and new maintenance routine. I would try and clear as much as possible in one go, an old credit card is my preferred algae scrapper, but if it's very tough you might need a razer blade. Make sure the new lights aren't on too long/too bright always best to start with a lower setting and gradually increase to avoid encouraging the algae.
I wouldn't bother with changing the substrate - sand doesn't degrade, it's just as good as it was 8 years ago... or more. Just breaking down a tank that's 20+ years (seal has gone so no way around it), will save the sand/gravel to reuse. You might find root tabs helpful though, as crypts have big root systems.