Change permalinks to category/postname – and set 301

Hi there,

I’ve been researching for answers for 2 hours now, so I came to the conclusion it would be better to ask here first before I become desperate ??

I’d like to change our permalink structure from

%postname%
to
%category%/%postname%

(site: https://www.studienstrategie.de)

Actually it’s no big deal, but since we habe about 80 posts, the manual configuration of 301 redirects would be quite complex.

I also figured out that it’s hard to tell the server that posts, which had no categories before, now have one as Apache doesn’t know the category of a post.

Is there a way to fix this or would it be easier to manually redirect all existing posts once I changed the permalinks? How to set up the according 301ers in the htaccess?

Thank you very much in advance.
Kind regards
Kargo