Permalink structure

Pretty risky.
I would recommend changing the permalink, then adding 301s for your existing posts.

The permalink structure is applied globally – so without some manipulaiton you cannot do it.

Some ideas:
– create a re-write rule + a regex for your existing posts:
https://developer.wordpress.org/reference/functions/add_rewrite_rule/
– assign a particular Tag to your old posts, then may be easier to re-write

Some reading:
https://gretchenlouise.com/update-old-posts/