Permalinks "listing" want other word

Hi, I saw that my permallnks are http://www.website-name.nl./listing/name-of-listing
But for the word “listing” I want an other word. So I looked in the permalinks and saw I could do it here. But it gives a warning, that when I change that I get 404 error. Now is it a test site and I already imported 563 from another access database. And think I cannot change the permalinks anymore now.

Anyone a suggestion about this. I know I can export my listing. And is it something to do that I remove all listings, set the permalink and then import the exported file?

Or can I make the permalink change now because it just a test site, nothing is on google index jet.