Interesting thing I tried on a test site:
I switched permalink structure from date/postname to postID/postname. From WordPress Permalinks panel – no plugins, no .htaccess entries, nothing, just switched permalink structure.
Then tried old post links, using the date/postname link. They were automatically redirected to the new address (postID/postname). I see no changes to .htaccess (made by WordPress).
Any idea how WordPress implements this?
Does it affect performance?
Read more here:: Permalink change redirects by WordPress