LibreCAD doesn't recalculate the drawing to a new unit. You have to do this yourself manually. If a length has been 10 inches it will now be 10 mm. Only the naming and according settings for dimensions are changed. You have to rescale the whole drawing with factor x25.4.
See here for the procedure of conversion in the other direction: https://dokuwiki.librecad.org/doku.php/usage:howto#how_to_convert_from_mm_to_inches