Permalinks – how to change the structure

Hi All,
I have just upgraded my site to the streamline theme which has categories and sub categories, unlike the my previous website.

All my SEO work is for https://www.mywebsite.com/thepage

so very simple, clean and tidy. Adding the caterogies now changes it to
https://www.mywebsite/thecatergory/thepage

which is going to mess up all my backlinks and SEO work.

I have checked with the settings and permalinks and I believe it is possible to change it back to original URL’s with the custom structure but I’m struggling to see how.

Its currently on /%category%/%postname% in the custom structure, I have played around with this a little but can’t seem to be able to take out the ‘thecatergory’ part or event edit this park when trying to change the permalink on the page edit.

This is really important do to previous back links so if anyone has any ideas or ways to change it, I would be really really grateful.

Many thanks.