easiest way would be to run some code through to correct – since you are asking, I expect this is beyond your skillset so suggest you post a request on
http://jobs.wordpress.net/
It should be a fairly cheap solution – code would be less than 20 lines !
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/.
Good job @phatmass … however all my users have nicknames 🙂
And the plugin changes the nicename (slug) into firstname-lastname and I just want to get rid of the space in the nicename field.
The plugin sure can do it … have to digg into the php/sql commands !