If you check the available methods at an XhtmlString property there are some ToHtml-methods you can use. Or are you using Web Forms?
If I recall correctly, your "external" link (i.e a link to an internal page, but entered in the "external page" field) WILL stay correct after publishing the page, UNLESS you enter the Insert link dialog again. If you do, the dialog will interpret the link as internal, and you will lose any special URL parameters etc that was stored in the link originally.
So the question is really what you want to do with the link (code-wise) after publishing the page, and whether editors must be able to edit the link after publishing.
I know that you should not mess with epis conversiont and routing BUT I need to to be able to fix a problem with a site.
Does anyone have a "good" fix for how I can disable epis automatic convertion from an external link to an internal link, if found internaly?
The long way to go is to make new blocks and replace all of them and put special properties in pages and so on. I would like to just disable this epi-routing.