Updating theme's translation-files

Hi,

I have translated a theme in finnish, so I have fi.mo and fi.po -files in use. Now when the theme’s author releases a new version of the theme then there are some changes in the theme’s dictionary and they are then missing in my translation-file. Is there any way to “update” my translation file so that it gets the new missing translations to it? Because I’d want to keep on using the translations I have already made before.