The original urls for users look like /author/user-name/
Is it possible to replace login by user's id? (something like: /author/34/ (34 is user id)).
I found the solution given here, to not work for my site.
it's simpler to do this via your theme's functions.php or a plugin rather than .htaccess:
make sure to visit the permalinks settings page after adding this to flush rewrite rules.