Apache Rewrite如何通过替换部分来重定向url？

Take this url for example:

 www.mysite.com/old/blablabla/some.html

I want to replace any request uri starting with /old/ with /new/old/. Then that sample should be

 www.mysite.com/new/old/blablabla/some.html

I tried this but I got 404：

RewriteCond %{REQUEST_URI} ^/old/
RewriteRule ^/old/(.*)$ /new/old/$1 [L]

Just with this:

RewriteRule ^/old/(.*) /new/old/$1 [L]

but /new/old/blablabla/some.html has to exist phisicaly in your server path.