I had been while using following permalink structure on my small wordpress blog formerly

http://domain.com/some-text-missing.html

Now I've transformed the permalink to

http://domain.com/archives/2010/09/some-text-missing/

The tag structure is

http://domain.com/archives/tag/sometag/

I'm while using wordpress redirection wordpress plugin and i'm tied to the standard expressions.

I'm not sure concerning the wordpress plugin you're using, but when you will find to not many sites, i'd perform the redirects directly inshtaccess such as this:

Redirect 301 /some-text-missing.html /archives/2010/09/some-text-missing/

you cannot use here a regex since the new url structure includes a new variable: the entire yearOr30 days. so regex wont know where you can redirect your customers. I'm not sure regarding your wordpress plugin, however i would write it myself, I'd browse the file title "some-text-missing" then search for this within the database and when found redirect towards the new url.