Yes it will effect.
when you will change you permalink, the backlinks for the old address will be dead.
You should about how many backlinks you have to these posts, because good permalink is kind of basic thing.

If you have good results at Google without nice permalinks, you should leave it like that, but if you are stuck I think you should consider that.

If you are using wordpress and for now your links is look like this "yourdomain.com/p=1" and you will change your permalink to "yourdomain.com/post-title" than your first and the second link will be same if you enter to the both of them manually you will be redirected to the "yourdomain.com/post-title"

Why bother? Keywords in the URL cannot be that important.

Keyword in URL is very important for SEO

Yes it will affect. But you can fix this without hurting your client's site.

There is one Worpress Plugin: WordPress › Simple 301 Redirects « WordPress Plugins

Simply redirect all old URL to properly formed URL. This way old URL won't lose any link juice and it will be passed to new URL.

Also make sure to create one Google Webmaster account and verify both the site so that Google come to know about the changes.

Hope that helps.

Regards,
Chandan

Best way of changing the url structure and to keep the old PR is to use 301 redirection techniques. There are low chances if somebody had pasted the permalink on their site as in most of the case the main url is shared.

You should definately do 301 redirects to new urls. It will pass the link juice.
Google respects 301 as far as I know.

Use a 301 redirect so that search engines know that page has been moved to a new url structure. Add the site to Google Webmaster tool to keep a track before proceeding.