At the moment I have a WordPress site which has some posts and the links for those look like this:
http://www.example.com/2015/09/some-post
I need to change those URL's to
http://www.example.com/some-post
which is easy enough but I want also to make the old links redirect to the new links automatically. Is there a way to do this with a plugin or in the .htaccess file or in some other way?
Yes, certainly you can do it either way. Via .htaccess redirect, or using a redirection plugin. Or you can use both at the same time.
However, assuming you are doing this for SEO purposes, and want to redirect previously linked URLs in order to pass any existing "link juice" to the new link, then you really should consider the plugin approach first.
The best feature of these plugins is the overall logging, mapping, and administration of your URL redirection plan. You will want to make sure all your old URLS are returning an HTTP 301 Moved Permanently response code, and not a 302 Moved Temporarily, or worse a 404. These plugins are great for this type of redirection task, and will quickly allow you to see if your old links are being redirected properly. Incorrect redirection of old links will result in loss of the link power/equity you may already have on those links.
