Rewrite already rewritten url

Hello,
I'm trying to rewrite user's profile url after it has been already rewritten by Joomla.

Htaccess rule:
RewriteCond %{REQUEST_URI} /mio-profile/userprofile/([a-zA-Z0-9_]+)$
RewriteRule ^my-profile/userprofile/([a-zA-Z0-9_]+)$ /${lowercase:$1}/ [L,R=301]

E.g.
I'm rewriting /my-profile/userprofile/username to /username/

The rewriting is working, but I'm getting error 404, page not found. Why?

Thank you.

(I know I can use 3rd party components such as 404sef, but I don't like them, I need rewrite only this kinda of url)

It's not working because there's no handler for it. PHP has no idea what that URL means. What happens when going to a page that isn't a user profile? How should Joomla behave? What if the username matches a component or other menu item; go to the user or to the component? Realistically such URLs just don't work well with Joomla as the alias (mio-profile) is what links it to the component followed by task/view. Your best solution is a 3rd party SEF extension that handles the re-routing for you, but you still may run into conflict issues.

Ah, thank you.

Can I do the opposite, then?
I mean rewrite /username/ to /my-profile/userprofile/username which should exists because it's written by Joomla?

Yes, a write for what you want it to be to what it should be or a permanent redirect in htaccess should work fine. You're going to have massive conflicts though and all URLs for yoursite.com/TEXT/ will try to go to a CB profile.

Right, maybe there is a way to check if the url /my-profile/userprofile/username/ exists? Else I don't redirect the url to it...

Custom 404 page can also work. This way valid links go to where they're supposed to. Invalid links go to 404 and attempts to see if the link is a user (using PHP). If it is then redirect and if not then display a typical 404 error page.