Hello,
Thank you for contacting the support and sorry for any inconvenience that might have been caused due to that.
If you’re changing your website’s general permalink structure, WordPress will automatically redirect the URL of your old post to the new one without any extra effort from your side. If for some reason, WordPress doesn’t automatically redirect your old URLs to the new ones, please make use of this regex to redirect year and month permalink to %postname%:
Source: /index.php/([0-9]{4})/([0-9]{2})/(.*)
Type: Regex
Destination: https://yourdomain.com/$3
The old link of yours should now redirect to the new permalink structure.
Let me know if it works on your end.
We are looking forward to helping you with this one.
Hello. I came back to this late.
The code didn’t work for me. What I did was to apply the redirection and I changed my permalink structure to postname alone. But all my url resulted in 404.
After changing my permalink to it’s initial custom format, urls are active even when the redirection is to on active status. This means that the redirection doesn’t work.

Hello,
We might need to take a closer look at the settings. Please edit the first post on this ticket and include your WordPress login in the designated Sensitive Data section.
Please do take a complete backup of your website before sharing the information with us.

It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead:
https://wordpress.org/plugins/temporary-login-without-password/
You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any):
https://wordpress.org/plugins/wp-security-audit-log/
We really look forward to helping you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.