Changed Permalink style –now broken?

Hello,
I’m still tinkering with my new blog, trying to learn the nuances. Everyone here has been a huge help!

So I changed my permalink structure from the default to “date and name based”.
Then from my main page, my “categories” and “pages” links were all broken (“page not found”). Is this because I changed the Permalink style? Seems that way as I switched it back and now they seem to be working.

(Options>Permalinks>Customize Permalink Structure)

If that was the case, how do I change my Permalink style AND make it backwards compatible with my previous posts?

THANKS!