Old urls

I have changed several of my pages on my site while for SEO purposes i.e. keywords. At the top of the page where it says Permalinks I have addedthe keyword.
Somebody who went on my site and found my listing clicked on it and instead of going to the new page it has gone to a 404. Please can you tell me how to get round this if it is already listed. Is it possible to get rid of 404 pages altogether and if so how? Many thanks.