WordPress Permalink Changes Causes Google Crawl Errors, Lowers Site Ranking

My apologies if this is already answered. I searched but could not find an answer.

My URL is: https://www.pixelhose.com

I changed the format of the Permalinks on my site to make the links more “SEO-friendly.” There were nearly 400 posts already on the site so instead of manually changing every reference, I used a plugin that traps bad URLs (Old Format) and re-writes them as New Format. My readers never notice a change. However, Google crawler flags these pages as 404 errors. That has caused Google to see a huge rise in not-found pages, causing negative consequences.

I have not found any fixes beyond manually “fixing” all the links and starting over with Google to re-build the site’s ranking. Any suggestions that helps me avoid that much work would be greatly appreciated.

This is a production site. Please note that while I do not believe upgrading to a newer version of either WordPress or the Theme would solve this issue, I had been forced to make code-level changes to the source code to fix some behavioral anomalies to implement some features for the site and, therefore, upgrading is not an option at this time.

Any and all help would be greatly appreciated.

Thanks,

Bob