I am changing my permalink structure from /%year%/%monthnum%/%postname%/ to /%category%/%postname%/.
Is it possible to redirect old links using old permalink structure to the new link? Possibly via the postname?
How to redirect to correct pages after permalink structure change
WordPress should handle all redirections correctly, without you needing to worry about them. For example, if your old post was http://example.com/2012/10/11/i-like-stackexchange any links should auto-magically be redirected to http://example.com/favorite-things/i-like-stackexchange. I just tested it on a site with WordPress 3.4 installed, and it worked just fine. WordPress can read your mind.
When you change your permalinks WordPress doesn’t always catch all the URLs that were changed. Luckily, there is a code solution from yoast: https://yoast.com/research/permalink-helper.php
Enter the old structure, new structure, and it outputs redirect code.
Works every time I used it.