Moving blog url – errors

I’m trying to move my blog from blog.forthelost.org to https://www.forthelost.org/blog/ I changed the URL in the system, but it just gives me a 404 message. The old URL works, but now won’t let me login from it, and when I try to change the URL back it claims it’s invalid. I’ve looked at the config but haven’t seen anything that I can modify to redirect the site.