Only rewriting guests or not rewriting internal links

Is it possible only rewriting guests? (Google would definitly not mind as long as it is only the url and not the content that is rewritten).

The main problem is that after you do the rewrite and users start posting internal links then you can never change rewrite rules at a later stage, without those links becomming invalid. Therefore it would be better to keep internal linking not rewritten. Is there any solution for this?

The solution is to NOT change your rewrite rules. This is a 'set it and forget it' option. It should NOT be changed after initial configuration.