you must remember autocad's Parent>>Child relationship.....
IF you attempt to insert block, whatever of the NEW same named dimension style into the old drawing, it will take on the old style properties
IF you insert the OLD file into a new blank file with the NEW same named dimension style, it will take on the new properties.
short answer
insert old file into new file...make sure dimension style has SAME EXACT NAME....all the old dimension will assume the new style