The site is currently using the Parameter format, I want to change to the Directories format, but Im concerned about loosing SEO ranking etc, once I change the format to directories, will all parameter links redirect to the director version automatically? Or do I need to put something in place to do so? I don't want to have to redirect every page one by one.
Redirect won't happen automatically as this is not expected, WordPress will redirect any URL with a parameter in the end, to a similar URL correct page, in your case default language.
You can create manual redirect rules for each or general redirect rules via htaccess.
I suggest checking on the next ticket and the solution client used, please note we can not guarantee this code and solution so I suggest checking further with a developer or hiring one.