[@phatmass Participant 11 years, 11 months ago]

I actually took a chance on this plugin that had zero feedback:

https://wordpress.org/support/plugin/change-all-users-slug

I ran it on all 4700 members with one click. After a couple minutes the plugin said it couldn’t change anything because all the usernames matched the display names.

BUT… whatever it did, it FIXED the 404 errors! Now, all my users that have spaces in their names are taken to a profile page like this /john-smith/ instead of this /JohnSmith/.