Change of permalinks?

Hi all, hope someone can help.

Few years back when i setup my blog i thought having permalink set to posts without the parent category was cool ex: https://www.domain.com/post-about-me/. Well now i would like to add the parent categories to the urls https://www.domain.com/topic-1/sub-topic-1/post-about-me/.

I can change the permalinks in the admin area to /%category%/%postname%/ but then all the cached pages from google, social networks and links i’v posted earlier are inaccessible. Thats the problem.

Hope there is workaround this issue and I’v read about .htaccess redirect but didn’t quite understand how it works and since its the critical file wouldn’t like to risk changing it without the help.

Also, i’m running multisite network and permalinks are changed to:

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

# END WordPress

Thanks