my permalink was like this %postname%/ then i changed it like this /%post_id%/%postname%/ now the problem is that google bot still indexing postname and report it as not found, so I want to add a line to robots.txt to prevent bots from indexing %postname%/ and only index /%post_id%/%postname%/ thanks
Not sure if this is really a question for SO but here are some points.
Your best bet hear is to chase point no. 1, cause doing anything else will hurt your search engine rankings. Giving the search engines a 301 response tells them that your page has moved so they should update their links. You shouldn't loose ranking by using this method.
Hope that helps.